Convert Colour to Hex Value

public static class ColourExtension {
    public static String HexConvert(this Color C, string Mask = "RGBA") {
        string returnString = "";
        string mask = Mask.ToLower();
        foreach (char c in mask) {
            switch (c) {
                case 'r':
                    returnString += C.R.ToString("X2");
                    break;
                case 'g':
                    returnString += C.G.ToString("X2");
                    break;
                case 'b':
                    returnString += C.B.ToString("X2");
                    break;
                case 'a':
                    returnString += C.A.ToString("X2");
                    break;
                default:
                    break;
            }
        }
        return returnString;
    }
}