title: Finding "equivalent" color with opacity
body: <p>Say I have a background color with a "ribbon" running over it in another solid color. Now, I want the ribbon to be partially transparent to let some details blend through, but still keep the ribbon the "same color" over the background.</p>
  
  <p>Is there a way to (easily) determine, for a given opacity/alpha &lt; 100% of the ribbon color, what RGB values it should have to be identical to its color with 100% opacity over the background?</p>
  
  <p>Here's a picture. Background is <code>rgb(72, 28, 97)</code>, ribbon <code>rgb(45, 34, 70)</code>. I want a <code>rgba(r, g, b, a)</code> for the ribbon so that it appears identical to this solid color.</p>
  
  <p><img src="https://i.stack.imgur.com/1dWd5.png" alt="enter image description here"></p>