An iOS extension to UIColor to get a white point corresponding to a temperature.

UIColor-colorWithKelvin.swift

extension UIColor {
    
    // based on  http://www.zombieprototypes.com/?p=210 who looked at some data and did a bunch of curve fitting
    static func colorWithKelvin( kelvin: CGFloat) -> UIColor {
        let k = kelvin < 1000 ? 1000 : ( kelvin > 40000 ? 40000 : kelvin)
        
        func interpolate( value: CGFloat, a: CGFloat, b:CGFloat, c:CGFloat) -> CGFloat {
            return a + b*value + c*log(value)
        }
        
        var red,green,blue: CGFloat
        
        if k < 6600 {
            red = 255
            green = interpolate(k/100-2, a: -155.25485562709179, b: -0.44596950469579133, c: 104.49216199393888)
            if k < 2000 {
                blue = 0
            } else {
                blue = interpolate(k/100-10, a: -254.76935184120902, b: 0.8274096064007395, c: 115.67994401066147)
            }
        } else {
            red = interpolate( k/100-55, a: 351.97690566805693, b: 0.114206453784165, c: -40.25366309332127)
            green = interpolate(k/100-50, a: 325.4494125711974, b: 0.07943456536662342, c: -28.0852963507957)
            blue = 255
        }
        
        return UIColor(red: red/255, green: green/255, blue: blue/255, alpha: 1.0)
    }

    static func colorByMultiplying( a: UIColor, _ b: UIColor) -> UIColor {
        var ar = CGFloat(0)
        var ab = CGFloat(0)
        var ag = CGFloat(0)
        var aa = CGFloat(0)
        
        var br = CGFloat(0)
        var bb = CGFloat(0)
        var bg = CGFloat(0)
        var ba = CGFloat(0)
 
        if a.getRed(&ar, green: &ag, blue: &ab, alpha: &aa) &&
            b.getRed(&br, green: &bg, blue: &bb, alpha: &ba) {
                return UIColor(red: ar*br, green: ag*bg, blue: ab*bb, alpha: aa*ba)
        } else {
            // Couldn't work.
            return a
        }

    }
}