I had been reading about the flash gels on http://strobist.blogspot.com and I wanted to experiment a bit. So I gave it a try, purchased a $10 Strobist gel kit to obtain a uniform blue background for the following image:
You can see this setup:
By changing the intensity of the strobe on the background you can vary the saturation of the blue. You also have to be careful to avoid any light leaking from the main softbox or the reflector onto the background, as this rapidly introduces a white cast. This is why the softbox and the reflector are placed at an angle and why I place the background as far away as possible form the subject.
Oh, and if you decide to stick the gel between the flash window and its wide angle fresnel diffuser as I did above, do not use the flash at full power. The heat will undulate the gel!