UIButton Background Colour Programmatically

Question

I have created a series of 'translucent' UI elements using @IBDesignable where I set the background colour to white with a low, partial alpha.

This works for UIView but for UIButton the background stays clear and the text colour, while correctly appears as white at runtime, shows as the default blue in Xcode (IB).

What am I doing wrong here? If it works for UIView I do not understand why the same code does not work for UIButton.

import UIKit

@IBDesignable
class TranslucentButton: UIButton {
    override func drawRect(rect: CGRect) {
        backgroundColor = UIColor(red: 1, green: 1, blue: 1, alpha: 0.1)
        setTitleColor(UIColor.whiteColor(), forState: UIControlState.Normal)

        layer.cornerRadius = 4
        layer.masksToBounds = true
        layer.borderColor = UIColor.whiteColor().CGColor
        layer.borderWidth = 1
    }
}

@IBDesignable
class TransulucentUIView: UIView {
    override func drawRect(rect: CGRect) {
        backgroundColor = UIColor(red: 1, green: 1, blue: 1, alpha: 0.1)

        layer.cornerRadius = 4
        layer.masksToBounds = true
        layer.borderColor = UIColor.whiteColor().CGColor
        layer.borderWidth = 1
    }
}