【Swift】作成済みのUIColorのalpha値のみを変えたい ref: http://qiita.com/taji-taji/items/f3681d15a5f6d7a79e06

file0.swift

let redColor = UIColor.redColor()
let transparentRedColor = redColor.colorWithAlphaComponent(0.6)

file1.swift

let redColor = UIColor.redColor()
let transparentRedColor = redColor.setAlpha(alpha: 0.6)

file2.swift

import UIKit

extension UIColor {

    func setAlpha(alpha a: CGFloat) -> UIColor {
        var red: CGFloat = 1.0
        var green: CGFloat = 1.0
        var blue: CGFloat = 1.0
        var alpha: CGFloat = 1.0
        self.getRed(&red, green: &green, blue: &blue, alpha: &alpha)

        return UIColor(red: red, green: green, blue: blue, alpha: a)
    }

}