Allow creating custom NSColors

[andrewzah]

I was looking at the code and only found clearColor. However, I'd like the ability to use custom colors (to change the title bar on OSX).

I can implement this probably if you point me in the right direction.

[jdm]

If you add more methods to the NSColor trait that send messages like https://developer.apple.com/documentation/appkit/nscolor/1535804-colorwithred?language=objc then it should be possible.