CocuLabel is subclass of UILabel that changes text color optimaized back view.
not yet
var label = CocuLabel()
label.updateColor()
// you can set priority three colors.
// if all colors don't satisfy, automatic optimized color is set
label.setPriorityColor(UIColor.whiteColor(),
second: UIColor.blackColor(),
third : UIColor.greenColor())
label.updateColor()