Reference the Color constants cheatsheet in the class reference

(cherry picked from commit de490855671cbef3be63e26180900235f8cbc0e1)

doc/classes/Color.xml

@@ -8,6 +8,7 @@
 		You can also create a color from standardized color names by using [method @GDScript.ColorN] or directly using the color constants defined here. The standardized color set is based on the [url=https://en.wikipedia.org/wiki/X11_color_names]X11 color names[/url].
 		If you want to supply values in a range of 0 to 255, you should use [method @GDScript.Color8].
 		[b]Note:[/b] In a boolean context, a Color will evaluate to [code]false[/code] if it's equal to [code]Color(0, 0, 0, 1)[/code] (opaque black). Otherwise, a Color will always evaluate to [code]true[/code].
+		[url=https://raw.githubusercontent.com/godotengine/godot-docs/master/img/color_constants.png]Color constants cheatsheet[/url]
 	</description>
 	<tutorials>
 	</tutorials>