link from Color to Colors (#3494)

* link from Color to Colors

* from review

lib/ui/painting.dart

@@ -33,6 +33,11 @@ Color _scaleAlpha(Color a, double factor) {
 /// Color c1 = const Color(0xFFFFFF); // fully transparent white (invisible)
 /// Color c2 = const Color(0xFFFFFFFF); // fully opaque white (visible)
 /// ```
+///
+/// See also:
+///
+///  * [Colors](https://docs.flutter.io/flutter/material/Colors-class.html), which
+///    defines the colors found in the Material Design specification.
 class Color {
   /// Construct a color from the lower 32 bits of an [int].
   ///