title: Adjust alpha of UIColor
body: <p>I set a <code>UIColor</code> using rgb to a background of a <code>UILabel</code>. I'm trying to adjust the <code>alpha</code> only. How can I modify the alpha of an existing rgb <code>UIColor</code>?</p>
  
  <p><strong>Edit</strong></p>
  
  <p>Basically I have <code>UILabels</code> that have a set <code>UIColor</code> (using rgb), and I won't know what color the <code>UILabels</code> are. At a certain point, I will have to change the <code>labels</code> alpha`. How can I just change the labels color alpha?</p>