title: computing RGBA to match an RGB color
body: <p>If I have an RGB color, with 100% opacity.</p>
  
  <p>I want that same color (or close to it) with a transparent alpha channel.  I will paint the transparent color over a white background.</p>
  
  <p>How do I compute the RGBA color?</p>
  
  <p>I guess what I am asking is the <a href="https://stackoverflow.com/questions/2049230/convert-rgba-color-to-rgb">opposite of this question</a>.</p>