What is the advantage of using an enum over a class in this example?

I'm wondering what the advantage to using an enum over a class in this example is. I can sense that there is some kind of advantage, but I'm having trouble wrapping my head around it.

If I were to use the following code, I would be able to have a class called RGBColour that I could call to assign an instance of UIColour to a variable or constant.

class RGBColour {

    var red: CGFloat
    var blue: CGFloat
    var green: CGFloat
    var alpha: CGFloat

    init(red: CGFloat, blue: CGFloat, green: CGFloat, alpha: CGFloat){
        self.red = red
        self.blue = blue
        self.green = green
        self.alpha = alpha
    }

    func color() -> UIColor {

        return UIColor(red: red/255.0, green: green/255.0, blue: blue/255.0, alpha: alpha)
    }
}

let someColor = RGBColour(red: 100, blue: 19, green: 20, alpha: 1)
someColor.color()

I could create another such class for HSB.

The advantages I see for an enum here are:

1. You can combine both RGB and HSB colour definitions into 1 object. (This would probably have more value in another circumstance?)

2. You can call the enum without assigning it to a variable. But I don't understand where this would be useful?

Can you please elaborate a bit on how an enum is more helpful for us in this example?

Thanks!