Simple feedback loop for matching on- and off-screen colors using a webcam and Processing.
Make a color area on your screen (I used background() at fullscreen), put an object in front of your screen, point a webcam at the screen, and average a portion of the object and a portion of the screen. Slowly modify the screen until the webcam can't tell the difference.
It doesn't matter if your white balance is correct or not, but making sure you don't overexpose/underexpose any of the channels is important.
You could definitely speed up the matching if you iteratively approximated Newton's method-style, with bigger jumps when you're further from the right color.