title: How to calculate an RGB colour by specifying an alpha blending amount?
body: <p>I'm writing a colour picker that gets the pixel RGB values from wherever you point to on the screen.  I want to also have the option of specifying that the colour I picked already has an alpha value.  I'm just wondering how I can calculate the resulting colour.</p>
  
  <p>For example:</p>
  
  <p>The resulting pixel colour is 240,247,249 but I know that the original colour had a 10% opacity and was on top of a white (255,255,255) background.  What's the calculation to work out the original RGB values?</p>