I recently experimented shooting with gels to change the background color. I started out with a roll of dark gray seamless paper. Then I positioned the background light a few feet from the background, and covered it with a Rosco gel. Next I proceeded to shoot a series of musical instruments with the background light 2 stops lower than the main light. I also used a reflector for fill and a piece of black foam as a flag to block light from the main light to the background.
The photo on the left was shot with a dark blue gel on the background. The photo on the right was shot with a dark purple gel on the background and a light purple gel over the main light to give the guitar head stock some color. Images may be licensed from iStockPhoto, click here for the series.