diff --git a/lib/src/callable.dart b/lib/src/callable.dart
index 1046e6a4..284209fe 100644
--- a/lib/src/callable.dart
+++ b/lib/src/callable.dart
@@ -2,13 +2,9 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-import 'ast/sass.dart';
-
 export 'callable/built_in.dart';
 export 'callable/user_defined.dart';
 
 abstract class Callable {
   String get name;
-
-  ArgumentDeclaration get arguments;
 }
diff --git a/lib/src/callable/built_in.dart b/lib/src/callable/built_in.dart
index 8c8cd445..5d5be10e 100644
--- a/lib/src/callable/built_in.dart
+++ b/lib/src/callable/built_in.dart
@@ -9,12 +9,16 @@ import '../value.dart';
 typedef Value _Callback(List<Value> arguments);
 
 class BuiltInCallable implements Callable {
-  final _Callback callback;
-
   final String name;
-  final ArgumentDeclaration arguments;
+  final List<ArgumentDeclaration> overloads;
+  final List<_Callback> callbacks;
 
-  BuiltInCallable(
-      this.name, this.arguments, Value callback(List<Value> arguments))
-      : callback = callback;
+  BuiltInCallable(String name, ArgumentDeclaration arguments,
+      Value callback(List<Value> arguments))
+      : this.overloaded(name, [arguments], [callback]);
+
+  BuiltInCallable.overloaded(this.name, Iterable<ArgumentDeclaration> arguments,
+      Iterable<_Callback> callbacks)
+      : overloads = new List.unmodifiable(arguments),
+        callbacks = new List.unmodifiable(callbacks);
 }
diff --git a/lib/src/functions.dart b/lib/src/functions.dart
index 45c2fe64..f01030a7 100644
--- a/lib/src/functions.dart
+++ b/lib/src/functions.dart
@@ -19,18 +19,48 @@ void defineCoreFunctions(Environment environment) {
     var blue = arguments[2].assertNumber("blue");
 
     return new SassColor.rgb(
-        _percentageOrUnitless(red, 255, "red"),
-        _percentageOrUnitless(green, 255, "green"),
-        _percentageOrUnitless(blue, 255, "blue"));
+        _percentageOrUnitless(red, 255, "red").round(),
+        _percentageOrUnitless(green, 255, "green").round(),
+        _percentageOrUnitless(blue, 255, "blue").round());
   }));
 
+  environment.setFunction(new BuiltInCallable.overloaded("rgba", [
+    new ArgumentDeclaration([
+      new Argument("red"),
+      new Argument("green"),
+      new Argument("blue"),
+      new Argument("alpha")
+    ]),
+    new ArgumentDeclaration([new Argument("color"), new Argument("alpha")]),
+  ], [
+    (arguments) {
+      // TODO: support calc strings
+      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(
+          _percentageOrUnitless(red, 255, "red").round(),
+          _percentageOrUnitless(green, 255, "green").round(),
+          _percentageOrUnitless(blue, 255, "blue").round(),
+          _percentageOrUnitless(alpha, 1, "alpha"));
+    },
+    (arguments) {
+      var color = arguments[0].assertColor("color");
+      var alpha = arguments[0].assertNumber("alpha");
+
+      return color.change(alpha: _percentageOrUnitless(alpha, 1, "alpha"));
+    }
+  ]));
+
   environment.setFunction(new BuiltInCallable(
       "inspect",
       new ArgumentDeclaration([new Argument("value")]),
       (arguments) => new SassString(arguments.single.toString())));
 }
 
-int _percentageOrUnitless(SassNumber number, int max, String name) {
+num _percentageOrUnitless(SassNumber number, num max, String name) {
   num value;
   if (!number.hasUnits) {
     value = number.value;
@@ -41,5 +71,5 @@ int _percentageOrUnitless(SassNumber number, int max, String name) {
         '\$$name: Expected $number to have no units or "%".');
   }
 
-  return value.clamp(0, max).round();
+  return value.clamp(0, max);
 }
diff --git a/lib/src/value/color.dart b/lib/src/value/color.dart
index a257f164..dcb1f8fe 100644
--- a/lib/src/value/color.dart
+++ b/lib/src/value/color.dart
@@ -12,13 +12,19 @@ class SassColor extends Value {
   final int red;
   final int green;
   final int blue;
+  final double alpha;
 
-  SassColor.rgb(this.red, this.green, this.blue);
+  SassColor.rgb(this.red, this.green, this.blue, [double alpha])
+      : alpha = alpha ?? 1.0;
 
   /*=T*/ accept/*<T>*/(ValueVisitor/*<T>*/ visitor) => visitor.visitColor(this);
 
   SassColor assertColor([String name]) => this;
 
+  SassColor change({int red, int green, int blue, double alpha}) =>
+      new SassColor.rgb(red ?? this.red, green ?? this.green, blue ?? this.blue,
+          alpha ?? this.alpha);
+
   Value plus(Value other) {
     if (other is! SassNumber && other is! SassColor) return super.plus(other);
     throw new InternalException('Undefined operation "$this + $other".');
diff --git a/lib/src/visitor/perform.dart b/lib/src/visitor/perform.dart
index 3f38fb4b..6f5e01c4 100644
--- a/lib/src/visitor/perform.dart
+++ b/lib/src/visitor/perform.dart
@@ -766,9 +766,26 @@ class PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
     var named = triple.item2;
     var separator = triple.item3;
 
-    _verifyArguments(positional, named, callable.arguments, invocation.span);
+    int overloadIndex;
+    for (var i = 0; i < callable.overloads.length - 1; i++) {
+      try {
+        _verifyArguments(
+            positional, named, callable.overloads[i], invocation.span);
+        overloadIndex = i;
+        break;
+      } on SassRuntimeException catch (_) {
+        continue;
+      }
+    }
+    if (overloadIndex == null) {
+      _verifyArguments(
+          positional, named, callable.overloads.last, invocation.span);
+      overloadIndex = callable.overloads.length - 1;
+    }
 
-    var declaredArguments = callable.arguments.arguments;
+    var overload = callable.overloads[overloadIndex];
+    var callback = callable.callbacks[overloadIndex];
+    var declaredArguments = overload.arguments;
     for (var i = positional.length; i < declaredArguments.length; i++) {
       var argument = declaredArguments[i];
       positional.add(
@@ -776,7 +793,7 @@ class PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
     }
 
     SassArgumentList argumentList;
-    if (callable.arguments.restArgument != null) {
+    if (overload.restArgument != null) {
       var rest = positional.length > declaredArguments.length
           ? positional.sublist(declaredArguments.length)
           : const <Value>[];
@@ -789,8 +806,7 @@ class PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
       positional.add(argumentList);
     }
 
-    var result =
-        _addExceptionSpan(() => callable.callback(positional), invocation.span);
+    var result = _addExceptionSpan(() => callback(positional), invocation.span);
 
     if (argumentList == null) return result;
     if (named.isEmpty) return result;
diff --git a/lib/src/visitor/serialize.dart b/lib/src/visitor/serialize.dart
index 43b8d9e2..1ebab4d5 100644
--- a/lib/src/visitor/serialize.dart
+++ b/lib/src/visitor/serialize.dart
@@ -225,10 +225,17 @@ class _SerializeCssVisitor
 
   void visitColor(SassColor value) {
     // TODO(nweiz): Use color names for named colors.
-    _buffer.writeCharCode($hash);
-    _writeHexComponent(value.red);
-    _writeHexComponent(value.green);
-    _writeHexComponent(value.blue);
+    if (value.alpha == 1) {
+      _buffer.writeCharCode($hash);
+      _writeHexComponent(value.red);
+      _writeHexComponent(value.green);
+      _writeHexComponent(value.blue);
+    } else {
+      // TODO: support precision in alpha, make sure we don't write exponential
+      // notation.
+      _buffer.write(
+          "rgb(${value.red}, ${value.green}, ${value.blue}, ${value.alpha})");
+    }
   }
 
   void _writeHexComponent(int color) {