Creating a random color to a label that does not appear twice

I am working on my first project where I am trying to create a random color to a UILabel (when a button is pressed) that does not repeat itself twice in a row. I have read many questions and found codes that generates random numbers that does not appear twice in a row, however, when I "link" that random number to a UIColor and "link" that UIColor to a label the code no longer works. This code has no errors but it allows the same color to appear twice;

@IBOutlet var HelloWorld: UILabel!
@IBOutlet var MyButton: UIButton!

@IBAction func MyButton(sender: AnyObject) {
let randomNumber2 = Int(arc4random_uniform(5))

    if randomNumber2 == 0 {
        HelloWorld.textColor = UIColor.redColor()
    }
    else if randomNumber2 == 1 {
        HelloWorld.textColor = UIColor.blueColor()
    }
    else if randomNumber2 == 2 {
        HelloWorld.textColor = UIColor.yellowColor()
    }
    else if randomNumber2 == 3 {
        HelloWorld.textColor = UIColor.orangeColor()
    }
    else if randomNumber2 == 4 {
        HelloWorld.textColor = UIColor.blueColor()
    }

Here is a code I have tried to copy from another answer but it has errors;

var previousNumber: UInt32?

@IBAction func MyButton(sender: AnyObject) {

func randomNumber() -> UInt32 {
        var randomNumber = arc4random_uniform(5)
        while previousNumber == randomNumber {
            randomNumber = arc4random_uniform(5)
        }
        previousNumber = randomNumber
        return randomNumber
    }

    if randomNumber2 == 0 {
        HelloWorld.textColor = UIColor.redColor()
    }
    else if randomNumber2 == 1 {
        HelloWorld.textColor = UIColor.blueColor()
    }
    else if randomNumber2 == 2 {
        HelloWorld.textColor = UIColor.yellowColor()
    }
    else if randomNumber2 == 3 {
        HelloWorld.textColor = UIColor.orangeColor()
    }
    else if randomNumber2 == 4 {
        HelloWorld.textColor = UIColor.blueColor()
    }
}

The error says: Binary operator '==' cannot be applied to operands of type '() -> UInt32' and 'Int'