Fun with default arguments in Swift initialisers

Color.swift

import UIKit

struct Color {
    let color: UIColor
    init(r: CGFloat = 0.0,
         g: CGFloat = 0.0,
         b: CGFloat = 0.0,
         a: CGFloat = 1.0)
    {
        color = UIColor(red: r, green: g, blue: b, alpha: a)
    }
    init(w: CGFloat,
         a: CGFloat = 1.0)
    {
        color = UIColor(white: w, alpha: a)
    }
    init(h: CGFloat,
         s: CGFloat = 1.0,
         l: CGFloat = 1.0,
         a: CGFloat = 1.0)
    {
        color = UIColor(hue: h, saturation: s, brightness: l, alpha: a)
    }
}

let black = Color()
let transparent = Color(a: 0.0)
let red = Color(r: 1.0)
let green = Color(g: 1.0)
let blue = Color(b: 1.0)
let white = Color(r: 1.0, g: 1.0, b: 1.0)