Swift: Change colour of button when tapped
<p>I'm new to Swift and I'm trying to change the button colour on tap. When the button is tapped, it should change colour and when released it should go back to the original button colour. </p>

<p>An example of this is the calculator. When you tap a button, it changes from light grey to dark grey and when the user removes their finger off the button, it goes back to the original light grey colour. How do I go about doing that?</p>

<p>So far, I've only got this..</p>

<pre><code>@IBAction func changeButtonColourOnTouch(sender: UIButton) {

    zeroButton.backgroundColor = UIColor.blueColor()

}
</code></pre>

<p>The above code changes the button colour to blue but stays blue.</p>