An extension of the iOS class UIColor to derive web safe colors.

UIColor+WebSafe.swift

/// An extension to derive web safe colors.
extension UIColor {

    /// The nearest web safe color to the current color. All web safe colors have RGB
    /// component values of 0, 51, 102, 153, 204, or 255 (20% intervals).
    var webSafe: UIColor {
        var (r, g, b, a) = (CGFloat(), CGFloat(), CGFloat(), CGFloat())
        getRed(&r, green: &g, blue: &b, alpha: &a)

        let clusters: [CGFloat] = [0.0, 0.2, 0.4, 0.6, 0.8, 1.0]
        func quantize(_ component: CGFloat) -> CGFloat {
            return clusters.min(by: { abs($0 - component) < abs($1 - component) })!
        }

        r = quantize(r)
        g = quantize(g)
        b = quantize(b)

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

}