Colored gels work best on a black backdrop. It sounds backwards, but since black absorbs the light that’s hitting it any gel you throw on a it will saturate very nicely. An example is in the link below. The portrait for the cover of HI’s baskeball preview as shot on a black backdrop using a red gel on the background light. Gelling on a black background also makes for really nice gradation of whatever color you are using. If you use no gel and just put straight light on a black backdrop it will turn gray or white depending on intensity of the light.

http://tinyurl.com/c8v5s4