How do you use the appropriate color class for the current platform?
Solution 1:
Use conditional compilation and type aliases:
#if os(OSX)
typealias Color = NSColor
typealias Image = NSImage
#else
typealias Color = UIColor
typealias Image = UIImage
#endif
Then use Color instead of UIColor or NSColor:
self.gameView!.backgroundColor = Color(red: 0, green: 0.2, blue: 0.5, alpha: 1)
Edit 2016-01-17: As DDPWNAGE noted above, Apple has created SKColor
with basically the same definition.