title: Convert rgba colour definition string in LESS to color instance
body: <p>Suppose I've parsed my mixin parameters and colours are provided using <code>rgba</code> function.</p>
  
  <p>Now I would like to mix two of those colours, but LESS <code>mix</code> function requires parameter instances of type <code>color</code>.</p>
  
  <h2>What I tried</h2>
  
  <p>...and doesn't work</p>
  
  <ol>
  <li>calling <code>color("rgba(0,0,0,.5)")</code></li>
  <li><code>mix(@first, ~"@{second}")</code> where <code>@second</code> is a string like <code>rgba(0,0,0,0.5)</code></li>
  </ol>
  
  <p><strong>How do I convert to color?</strong></p>