The extension to return the complement colour or two complement colours of a colour in Swift.

UIColor+Extension

extension UIColor {
    convenience init(red: Int, green: Int, blue: Int) {
        let newRed = CGFloat(red)/255
        let newGreen = CGFloat(green)/255
        let newBlue = CGFloat(blue)/255
        self.init(red: newRed, green: newGreen, blue: newBlue, alpha: 1.0)
    }
    
    var complement: UIColor {
        return self.withHueOffset(0.5)
    }
    
    var splitComplement0: UIColor {
        return self.withHueOffset(150 / 360)
    }
    
    var splitComplement1: UIColor {
        return self.withHueOffset(210 / 360)
    }

    private func withHueOffset(_ offset: CGFloat) -> UIColor {
        var h: CGFloat = 0
        var s: CGFloat = 0
        var b: CGFloat = 0
        var a: CGFloat = 0
        self.getHue(&h, saturation: &s, brightness: &b, alpha: &a)
        return UIColor(hue: fmod(h + offset, 1), saturation: s, brightness: b, alpha: a)
    }
}