SwiftUI 2.0
There is a new initializer that takes a Color and returns a UIColor for iOS or NSColor for macOS now. So:
iOS
UIColor(Color.red)
macOS
NSColor(Color.red)
Core Graphics
UIColor(Color.red).cgColor /* For iOS */
NSColor(Color.red).cgColor /* For macOS */
If you are looking for color components, you can find a helpful extensions here in this answer
Also, check out How to convert UIColor to SwiftUI‘s Color