Having done some experimenting with White Balance and a single red-colored light, I decided to take things a step further -- a red-lit subject with a blue-lit background.

The best tool for the background would have been a blue gel on a flash, but not having a blue gel, I resorted to what I did have -- a blue light bulb. Unfortunately, a bulb like this gives off a very weak light, so when I shot this I had to have a pretty bright exposure to let in enough background light, but as you can see in the first shot it led to a very high exposure for the red light. Also, it was in Auto White Balance which in this shot was too warm -- the blue in the background was not blue enought.

Oh, and I'd say excuse the subject matter but I chose it because it was around and has detail.

In the second image (a copy) I brought down the exposure so the red tones down considerably to show detail and the background doesn't really suffer.

But the fun was in the third exposure -- I cooled the white balance way down, tweaked the red and blue channels a little, and look what happened to the background!