This is a quick and easy way to deal with severe colorshifts caused by bad film, light leaks, or radical lighting problems. It's not appropriate for irregular colorshifts. This is a coarse method, and will probably need fine-tuning afterwards.
Here's our sample image from our Archives. It's a cute picture, but obviously has a lot of problems, the primary one being the awful red/orange cast.
The first thing we do is to sample an area using the eyedropper set to 5x5. The idea is to sample an area that would actually be neutral if the picture were properly balanced. Next we make an empty layer, and fill it with the new foreground color.
Now we need to invert this solid fill color using CTRL-I or Image > Adjustments > Invert.
Finally, we set the blending mode of this inverted color layer to Overlay.
Some final adjustments, and we're done. It's far from perfect, but can turn a morale-busting impossible picture into a more manageable one.