Nullify cssString cache when color/alpha values change.

src/color/Color.js

@@ -14,6 +14,7 @@ Color = Base.extend({
 		else if (this._alpha < 0) this._alpha = 0;
 		else if (alpha > 1) this._alpha = 1;
 		else this._alpha = alpha;
+		this._cssString = null;
 	},
 	
 	/**

src/color/GrayColor.js

@@ -82,6 +82,7 @@ GrayColor = Color.extend({
 			return 1 - this._gray;
 		};
 		fields['set' + key.capitalize()] = function(value) {
+			this._cssString = null;
 			this._gray = this._gray * (1 - weight) + weight * (1 - value);
 		};
 	});

src/color/RGBColor.js

@@ -156,6 +156,7 @@ RGBColor = Color.extend(new function() {
 		},
 
 		setGray: function(gray) {
+			this._cssString = null;
 			this._red = this._green = this._blue = gray;
 		},