How can we use Assets Catalog Color Sets?

Question

I usually use custom UIColors on iOS using extensions with Swift, but now with iOS 11/ Xcode 9 we can create Colors Sets. How can we use them?

Update - Tip

Answer 1

 // iOS
 let color = UIColor(named: "SillyBlue")

 // macOS
 let color = NSColor(named: "SillyBlue")