An extension of the iOS class UIColor to provide conversion to and from CMYK (cyan, magenta, yellow, black) colors.

UIColor+CMYK.swift

/// An extension to provide conversion to and from CMYK (cyan, magenta, yellow, black) colors.
extension UIColor {

    /// The CMYK (cyan, magenta, yellow, black) components of a color, in the range [0, 100%].
    struct CMYK: Hashable {

        /// The cyan component of the color, in the range [0, 100%].
        var cyan: CGFloat
        /// The magenta component of the color, in the range [0, 100%].
        var magenta: CGFloat
        /// The yellow component of the color, in the range [0, 100%].
        var yellow: CGFloat
        /// The black component of the color, in the range [0, 100%].
        var black: CGFloat

    }

    /// The CMYK (cyan, magenta, yellow, black) components of the color, in the range [0, 100%].
    var cmyk: CMYK {
        var (r, g, b) = (CGFloat(), CGFloat(), CGFloat())
        getRed(&r, green: &g, blue: &b, alpha: nil)

        let k = 1.0 - max(r, g, b)
        var c = (1.0 - r - k) / (1.0 - k)
        var m = (1.0 - g - k) / (1.0 - k)
        var y = (1.0 - b - k) / (1.0 - k)

        if c.isNaN { c = 0.0 }
        if m.isNaN { m = 0.0 }
        if y.isNaN { y = 0.0 }

        return CMYK(cyan: c * 100.0,
                    magenta: m * 100.0,
                    yellow: y * 100.0,
                    black: k * 100.0)
    }

    /// Initializes a color from CMYK (cyan, magenta, yellow, black) components.
    /// - parameter cmyk: The components used to initialize the color.
    /// - parameter alpha: The alpha value of the color.
    convenience init(_ cmyk: CMYK, alpha: CGFloat = 1.0) {
        let c = cmyk.cyan / 100.0
        let m = cmyk.magenta / 100.0
        let y = cmyk.yellow / 100.0
        let k = cmyk.black / 100.0

        let r = (1.0 - c) * (1.0 - k)
        let g = (1.0 - m) * (1.0 - k)
        let b = (1.0 - y) * (1.0 - k)

        self.init(red: r, green: g, blue: b, alpha: alpha)
    }

}