diff --git a/CHANGELOG.md b/CHANGELOG.md
index 59365084..000ca8b7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,10 @@
 * Add support for passing arguments to `@content` blocks. See [the
   proposal][content-args] for details.
 
+* Add support for the new `rgb()` and `hsl()` syntax introduced in CSS Colors
+  Level 4, such as `rgb(0% 100% 0% / 0.5)`. See [the proposal][color-4-rgb-hsl]
+  for more details.
+
 * Add support for interpolation in at-rule names. See [the
   proposal][at-rule-interpolation] for details.
 
@@ -17,6 +21,7 @@
 * Properly compile selectors that end in escaped whitespace.
 
 [content-args]: https://github.com/sass/language/blob/master/accepted/content-args.md
+[color-4-rgb-hsl]: https://github.com/sass/language/blob/master/accepted/color-4-rgb-hsl.md
 [at-rule-interpolation]: https://github.com/sass/language/blob/master/accepted/at-rule-interpolation.md
 
 ### JavaScript API
diff --git a/lib/src/functions.dart b/lib/src/functions.dart
index 0fc1e547..924ae9ba 100644
--- a/lib/src/functions.dart
+++ b/lib/src/functions.dart
@@ -44,100 +44,26 @@ final List<BuiltInCallable> coreFunctions = new UnmodifiableListView([
   // ### RGB
 
   new BuiltInCallable.overloaded("rgb", {
-    r"$red, $green, $blue": (arguments) {
-      if (arguments[0].isSpecialNumber ||
-          arguments[1].isSpecialNumber ||
-          arguments[2].isSpecialNumber) {
-        return _functionString('rgb', arguments);
-      }
-
-      var red = arguments[0].assertNumber("red");
-      var green = arguments[1].assertNumber("green");
-      var blue = arguments[2].assertNumber("blue");
-
-      return new SassColor.rgb(
-          fuzzyRound(_percentageOrUnitless(red, 255, "red")),
-          fuzzyRound(_percentageOrUnitless(green, 255, "green")),
-          fuzzyRound(_percentageOrUnitless(blue, 255, "blue")));
-    },
-    r"$red, $green": (arguments) {
-      // rgb(123, var(--foo)) is valid CSS because --foo might be `456, 789` and
-      // functions are parsed after variable substitution.
-      if (arguments[0].isVar || arguments[1].isVar) {
-        return _functionString('rgb', arguments);
-      } else {
-        throw new SassScriptException(r"Missing argument $blue.");
-      }
-    },
-    r"$red": (arguments) {
-      if (arguments.first.isVar) {
-        return _functionString('rgb', arguments);
-      } else {
-        throw new SassScriptException(r"Missing argument $green.");
-      }
+    r"$red, $green, $blue, $alpha": (arguments) => _rgb("rgb", arguments),
+    r"$red, $green, $blue": (arguments) => _rgb("rgb", arguments),
+    r"$color, $alpha": (arguments) => _rgbTwoArg("rgb", arguments),
+    r"$channels": (arguments) {
+      var parsed = _parseChannels(
+          "rgb", [r"$red", r"$green", r"$blue"], arguments.first);
+      return parsed is SassString ? parsed : _rgb("rgb", parsed as List<Value>);
     }
   }),
 
   new BuiltInCallable.overloaded("rgba", {
-    r"$red, $green, $blue, $alpha": (arguments) {
-      if (arguments[0].isSpecialNumber ||
-          arguments[1].isSpecialNumber ||
-          arguments[2].isSpecialNumber ||
-          arguments[3].isSpecialNumber) {
-        return _functionString('rgba', arguments);
-      }
-
-      var red = arguments[0].assertNumber("red");
-      var green = arguments[1].assertNumber("green");
-      var blue = arguments[2].assertNumber("blue");
-      var alpha = arguments[3].assertNumber("alpha");
-
-      return new SassColor.rgb(
-          fuzzyRound(_percentageOrUnitless(red, 255, "red")),
-          fuzzyRound(_percentageOrUnitless(green, 255, "green")),
-          fuzzyRound(_percentageOrUnitless(blue, 255, "blue")),
-          _percentageOrUnitless(alpha, 1, "alpha"));
-    },
-    r"$color, $alpha": (arguments) {
-      // rgba(var(--foo), 0.5) is valid CSS because --foo might be `123, 456,
-      // 789` and functions are parsed after variable substitution.
-      if (arguments[0].isVar) {
-        return _functionString('rgba', arguments);
-      } else if (arguments[1].isVar) {
-        var first = arguments[0];
-        if (first is SassColor) {
-          return new SassString(
-              "rgba(${first.red}, ${first.green}, ${first.blue}, "
-              "${arguments[1].toCssString()})",
-              quotes: false);
-        } else {
-          return _functionString('rgba', arguments);
-        }
-      } else if (arguments[1].isSpecialNumber) {
-        var color = arguments[0].assertColor("color");
-        return new SassString(
-            "rgba(${color.red}, ${color.green}, ${color.blue}, "
-            "${arguments[1].toCssString()})",
-            quotes: false);
-      }
-
-      var color = arguments[0].assertColor("color");
-      var alpha = arguments[1].assertNumber("alpha");
-      return color.changeAlpha(_percentageOrUnitless(alpha, 1, "alpha"));
-    },
-    r"$red, $green, $blue": (arguments) {
-      if (arguments[0].isVar || arguments[1].isVar || arguments[2].isVar) {
-        return _functionString('rgba', arguments);
-      } else {
-        throw new SassScriptException(r"Missing argument $alpha.");
-      }
-    },
-    r"$red": (arguments) {
-      if (arguments.first.isVar) {
-        return _functionString('rgba', arguments);
-      } else {
-        throw new SassScriptException(r"Missing argument $green.");
-      }
+    r"$red, $green, $blue, $alpha": (arguments) => _rgb("rgba", arguments),
+    r"$red, $green, $blue": (arguments) => _rgb("rgba", arguments),
+    r"$color, $alpha": (arguments) => _rgbTwoArg("rgba", arguments),
+    r"$channels": (arguments) {
+      var parsed = _parseChannels(
+          "rgba", [r"$red", r"$green", r"$blue"], arguments.first);
+      return parsed is SassString
+          ? parsed
+          : _rgb("rgba", parsed as List<Value>);
     }
   }),
 
@@ -163,20 +89,9 @@ final List<BuiltInCallable> coreFunctions = new UnmodifiableListView([
   // ### HSL
 
   new BuiltInCallable.overloaded("hsl", {
-    r"$hue, $saturation, $lightness": (arguments) {
-      if (arguments[0].isSpecialNumber ||
-          arguments[1].isSpecialNumber ||
-          arguments[2].isSpecialNumber) {
-        return _functionString("hsl", arguments);
-      }
-
-      var hue = arguments[0].assertNumber("hue");
-      var saturation = arguments[1].assertNumber("saturation");
-      var lightness = arguments[2].assertNumber("lightness");
-
-      return new SassColor.hsl(hue.value, saturation.value.clamp(0, 100),
-          lightness.value.clamp(0, 100));
-    },
+    r"$hue, $saturation, $lightness, $alpha": (arguments) =>
+        _hsl("hsl", arguments),
+    r"$hue, $saturation, $lightness": (arguments) => _hsl("hsl", arguments),
     r"$hue, $saturation": (arguments) {
       // hsl(123, var(--foo)) is valid CSS because --foo might be `10%, 20%` and
       // functions are parsed after variable substitution.
@@ -186,44 +101,17 @@ final List<BuiltInCallable> coreFunctions = new UnmodifiableListView([
         throw new SassScriptException(r"Missing argument $lightness.");
       }
     },
-    r"$hue": (arguments) {
-      if (arguments.first.isVar) {
-        return _functionString('hsl', arguments);
-      } else {
-        throw new SassScriptException(r"Missing argument $saturation.");
-      }
+    r"$channels": (arguments) {
+      var parsed = _parseChannels(
+          "hsl", [r"$hue", r"$saturation", r"$lightness"], arguments.first);
+      return parsed is SassString ? parsed : _hsl("hsl", parsed as List<Value>);
     }
   }),
 
   new BuiltInCallable.overloaded("hsla", {
-    r"$hue, $saturation, $lightness, $alpha": (arguments) {
-      if (arguments[0].isSpecialNumber ||
-          arguments[1].isSpecialNumber ||
-          arguments[2].isSpecialNumber ||
-          arguments[3].isSpecialNumber) {
-        return _functionString("hsla", arguments);
-      }
-
-      var hue = arguments[0].assertNumber("hue");
-      var saturation = arguments[1].assertNumber("saturation");
-      var lightness = arguments[2].assertNumber("lightness");
-      var alpha = arguments[3].assertNumber("alpha");
-
-      return new SassColor.hsl(
-          hue.value,
-          saturation.value.clamp(0, 100),
-          lightness.value.clamp(0, 100),
-          _percentageOrUnitless(alpha, 1, "alpha"));
-    },
-    r"$hue, $saturation, $lightness": (arguments) {
-      // hsla(123, var(--foo)) is valid CSS because --foo might be `10%, 20%,
-      // 0.5` and functions are parsed after variable substitution.
-      if (arguments[0].isVar || arguments[1].isVar || arguments[2].isVar) {
-        return _functionString('hsla', arguments);
-      } else {
-        throw new SassScriptException(r"Missing argument $alpha.");
-      }
-    },
+    r"$hue, $saturation, $lightness, $alpha": (arguments) =>
+        _hsl("hsla", arguments),
+    r"$hue, $saturation, $lightness": (arguments) => _hsl("hsla", arguments),
     r"$hue, $saturation": (arguments) {
       if (arguments[0].isVar || arguments[1].isVar) {
         return _functionString('hsla', arguments);
@@ -231,12 +119,12 @@ final List<BuiltInCallable> coreFunctions = new UnmodifiableListView([
         throw new SassScriptException(r"Missing argument $lightness.");
       }
     },
-    r"$hue": (arguments) {
-      if (arguments.first.isVar) {
-        return _functionString('hsla', arguments);
-      } else {
-        throw new SassScriptException(r"Missing argument $saturation.");
-      }
+    r"$channels": (arguments) {
+      var parsed = _parseChannels(
+          "hsla", [r"$hue", r"$saturation", r"$lightness"], arguments.first);
+      return parsed is SassString
+          ? parsed
+          : _hsl("hsla", parsed as List<Value>);
     }
   }),
 
@@ -1009,6 +897,135 @@ SassString _functionString(String name, Iterable<Value> arguments) =>
             ")",
         quotes: false);
 
+Value _rgb(String name, List<Value> arguments) {
+  var alpha = arguments.length > 3 ? arguments[3] : null;
+  if (arguments[0].isSpecialNumber ||
+      arguments[1].isSpecialNumber ||
+      arguments[2].isSpecialNumber ||
+      (alpha?.isSpecialNumber ?? false)) {
+    return _functionString(name, arguments);
+  }
+
+  var red = arguments[0].assertNumber("red");
+  var green = arguments[1].assertNumber("green");
+  var blue = arguments[2].assertNumber("blue");
+
+  return new SassColor.rgb(
+      fuzzyRound(_percentageOrUnitless(red, 255, "red")),
+      fuzzyRound(_percentageOrUnitless(green, 255, "green")),
+      fuzzyRound(_percentageOrUnitless(blue, 255, "blue")),
+      alpha == null
+          ? null
+          : _percentageOrUnitless(alpha.assertNumber("alpha"), 1, "alpha"));
+}
+
+Value _rgbTwoArg(String name, List<Value> arguments) {
+  // rgba(var(--foo), 0.5) is valid CSS because --foo might be `123, 456, 789`
+  // and functions are parsed after variable substitution.
+  if (arguments[0].isVar) {
+    return _functionString(name, arguments);
+  } else if (arguments[1].isVar) {
+    var first = arguments[0];
+    if (first is SassColor) {
+      return new SassString(
+          "$name(${first.red}, ${first.green}, ${first.blue}, "
+          "${arguments[1].toCssString()})",
+          quotes: false);
+    } else {
+      return _functionString(name, arguments);
+    }
+  } else if (arguments[1].isSpecialNumber) {
+    var color = arguments[0].assertColor("color");
+    return new SassString(
+        "$name(${color.red}, ${color.green}, ${color.blue}, "
+        "${arguments[1].toCssString()})",
+        quotes: false);
+  }
+
+  var color = arguments[0].assertColor("color");
+  var alpha = arguments[1].assertNumber("alpha");
+  return color.changeAlpha(_percentageOrUnitless(alpha, 1, "alpha"));
+}
+
+Value _hsl(String name, List<Value> arguments) {
+  var alpha = arguments.length > 3 ? arguments[3] : null;
+  if (arguments[0].isSpecialNumber ||
+      arguments[1].isSpecialNumber ||
+      arguments[2].isSpecialNumber ||
+      (alpha?.isSpecialNumber ?? false)) {
+    return _functionString(name, arguments);
+  }
+
+  var hue = arguments[0].assertNumber("hue");
+  var saturation = arguments[1].assertNumber("saturation");
+  var lightness = arguments[2].assertNumber("lightness");
+
+  return new SassColor.hsl(
+      hue.value,
+      saturation.value.clamp(0, 100),
+      lightness.value.clamp(0, 100),
+      alpha == null
+          ? null
+          : _percentageOrUnitless(alpha.assertNumber("alpha"), 1, "alpha"));
+}
+
+/* SassString | List<Value> */ _parseChannels(
+    String name, List<String> argumentNames, Value channels) {
+  if (channels.isVar) return _functionString(name, [channels]);
+
+  var isCommaSeparated = channels.separator == ListSeparator.comma;
+  var isBracketed = channels.hasBrackets;
+  if (isCommaSeparated || isBracketed) {
+    var buffer = new StringBuffer(r"$channels must be");
+    if (isBracketed) buffer.write(" an unbracketed");
+    if (isCommaSeparated) {
+      buffer.write(isBracketed ? "," : " a");
+      buffer.write(" space-separated");
+    }
+    buffer.write(" list.");
+    throw new SassScriptException(buffer.toString());
+  }
+
+  var list = channels.asList;
+  if (list.length > 3) {
+    throw new SassScriptException(
+        "Only 3 elements allowed, but ${list.length} were passed.");
+  } else if (list.length < 3) {
+    if (list.any((value) => value.isVar) ||
+        (list.isNotEmpty && _isVarSlash(list.last))) {
+      return _functionString(name, [channels]);
+    } else {
+      var argument = argumentNames[list.length];
+      throw new SassScriptException("Missing element $argument.");
+    }
+  }
+
+  var maybeSlashSeparated = list[2];
+  if (maybeSlashSeparated is SassNumber &&
+      maybeSlashSeparated.asSlash != null) {
+    return [
+      list[0],
+      list[1],
+      maybeSlashSeparated.asSlash.item1,
+      maybeSlashSeparated.asSlash.item2
+    ];
+  } else if (maybeSlashSeparated is SassString &&
+      !maybeSlashSeparated.hasQuotes &&
+      maybeSlashSeparated.text.contains("/")) {
+    return _functionString(name, [channels]);
+  } else {
+    return list;
+  }
+}
+
+/// Returns whether [value] is an unquoted string that start with `var(` and
+/// contains `/`.
+bool _isVarSlash(Value value) =>
+    value is SassString &&
+    value.hasQuotes &&
+    startsWithIgnoreCase(value.text, "var(") &&
+    value.text.contains("/");
+
 /// Asserts that [number] is a percentage or has no units, and normalizes the
 /// value.
 ///
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index 99ef99c3..829a5a98 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -236,6 +236,18 @@ bool equalsIgnoreCase(String string1, String string2) {
   return string1.toUpperCase() == string2.toUpperCase();
 }
 
+/// Returns whether [string] starts with [prefix], ignoring ASCII case.
+bool startsWithIgnoreCase(String string, String prefix) {
+  if (string.length < prefix.length) return false;
+  for (var i = 0; i < prefix.length; i++) {
+    if (!characterEqualsIgnoreCase(
+        string.codeUnitAt(i), prefix.codeUnitAt(i))) {
+      return false;
+    }
+  }
+  return true;
+}
+
 /// Returns an empty map that uses [equalsIgnoreSeparator] for key equality.
 ///
 /// If [source] is passed, copies it into the map.
diff --git a/lib/src/value/number.dart b/lib/src/value/number.dart
index c314a231..b9ea1c8a 100644
--- a/lib/src/value/number.dart
+++ b/lib/src/value/number.dart
@@ -4,6 +4,8 @@
 
 import 'dart:math';
 
+import 'package:tuple/tuple.dart';
+
 import '../exception.dart';
 import '../util/number.dart';
 import '../utils.dart';
@@ -151,8 +153,9 @@ class SassNumber extends Value implements ext.SassNumber {
 
   final List<String> denominatorUnits;
 
-  /// The slash-separated representation of this number, if it has one.
-  final String asSlash;
+  /// The representation of this number as two slash-separated numbers, if it
+  /// has one.
+  final Tuple2<SassNumber, SassNumber> asSlash;
 
   bool get hasUnits => numeratorUnits.isNotEmpty || denominatorUnits.isNotEmpty;
 
@@ -188,9 +191,11 @@ class SassNumber extends Value implements ext.SassNumber {
     return new SassNumber._(value, numeratorUnits, denominatorUnits);
   }
 
-  /// Returns a copy of [this] with [this.asSlash] set to [asSlash].
-  SassNumber withSlash(String asSlash) =>
-      new SassNumber._(value, numeratorUnits, denominatorUnits, asSlash);
+  /// Returns a copy of [this] with [this.asSlash] set to a tuple containing
+  /// [numerator] and [denominator].
+  SassNumber withSlash(SassNumber numerator, SassNumber denominator) =>
+      new SassNumber._(value, numeratorUnits, denominatorUnits,
+          new Tuple2(numerator, denominator));
 
   SassNumber assertNumber([String name]) => this;
 
diff --git a/lib/src/visitor/async_evaluate.dart b/lib/src/visitor/async_evaluate.dart
index cfb559ae..f696adc8 100644
--- a/lib/src/visitor/async_evaluate.dart
+++ b/lib/src/visitor/async_evaluate.dart
@@ -1190,9 +1190,7 @@ class _EvaluateVisitor
           var right = await node.right.accept(this);
           var result = left.dividedBy(right);
           if (node.allowsSlash && left is SassNumber && right is SassNumber) {
-            var leftSlash = left.asSlash ?? _serialize(left, node.left.span);
-            var rightSlash = right.asSlash ?? _serialize(right, node.left.span);
-            return (result as SassNumber).withSlash("$leftSlash/$rightSlash");
+            return (result as SassNumber).withSlash(left, right);
           } else {
             return result;
           }
diff --git a/lib/src/visitor/evaluate.dart b/lib/src/visitor/evaluate.dart
index 8ce9559e..77aeb3ff 100644
--- a/lib/src/visitor/evaluate.dart
+++ b/lib/src/visitor/evaluate.dart
@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_evaluate.dart.
 // See tool/synchronize.dart for details.
 //
-// Checksum: abeba8d186777d0dc1eedf9f9d29e4514bcd7619
+// Checksum: 6da6661213e8c929ae91a0a993a6cf2827f033f5
 
 import 'async_evaluate.dart' show EvaluateResult;
 export 'async_evaluate.dart' show EvaluateResult;
@@ -1183,9 +1183,7 @@ class _EvaluateVisitor
           var right = node.right.accept(this);
           var result = left.dividedBy(right);
           if (node.allowsSlash && left is SassNumber && right is SassNumber) {
-            var leftSlash = left.asSlash ?? _serialize(left, node.left.span);
-            var rightSlash = right.asSlash ?? _serialize(right, node.left.span);
-            return (result as SassNumber).withSlash("$leftSlash/$rightSlash");
+            return (result as SassNumber).withSlash(left, right);
           } else {
             return result;
           }
diff --git a/lib/src/visitor/serialize.dart b/lib/src/visitor/serialize.dart
index 5731ad72..cb4e42b1 100644
--- a/lib/src/visitor/serialize.dart
+++ b/lib/src/visitor/serialize.dart
@@ -644,7 +644,9 @@ class _SerializeVisitor implements CssVisitor, ValueVisitor, SelectorVisitor {
 
   void visitNumber(SassNumber value) {
     if (value.asSlash != null) {
-      _buffer.write(value.asSlash);
+      visitNumber(value.asSlash.item1);
+      _buffer.writeCharCode($slash);
+      visitNumber(value.asSlash.item2);
       return;
     }
 
diff --git a/pubspec.yaml b/pubspec.yaml
index 9fdb64ad..0e011578 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: sass
-version: 1.15.0-dev
+version: 1.15.0
 description: A Sass implementation in Dart.
 author: Dart Team <misc@dartlang.org>
 homepage: https://github.com/sass/dart-sass