 <p>Heres a tutorial-ish thing:</p> <p><a href="https://robots.thoughtbot.com/designing-for-ios-blending-modes" rel="nofollow">Blending Modes in iOS</a></p> <p>And here is the <a href="https://developer.apple.com/library/ios/documentation/GraphicsImaging/Reference/CGContext/index.html#//apple_ref/doc/c_ref/CGBlendMode" rel="nofollow">Apple Documentation</a> that covers blend modes. Should get you on the right track. The exact formulas are shown for each blend mode option.</p> <p>Finally, see this answer:</p> <p><a href="http://stackoverflow.com/a/8354632/2079103">How to get the color of a displayed pixel</a></p>