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Exactly like Paul said. However, many do not realize the effect of the main and fill lights on the background. You need to have enough distance behind your subject that the main and fill lights do not affect them. This means having a lot of room, or compressing the main and fill closer to the subject.
The distances involved are not absolute numbers, but ratios of main/fill to subject vs. subject to background. If your lights are 1' from your subject, and your subject is 3' from the background, your BG gets 4 stops less light (or 1/16 the amount). If you move the lights back to 3' from your subject (you will have to increase power of the light, or change aperture to compensate for light on your subject), the BG is 2 stops less bright, even though it is the same distance to the subject. Now, if you move your subject out to 9' from the background, and the lights stay 3' from the subject, the BG is back to 4 stops less light.
This follows the inverse square law. In the first example, the light travels 1' to the subject for a given exposure. At one foot behind the subject, the light has traveled two total feet, twice the distance to the subject. The inverse square law states this is 1/4 the intensity (2 stops). At 3' behind the subject, the light has doubled again, causing another drop in intensity by 2 stops.
In the second example, the light only doubled once, 3' to the subject, another 3' to the BG, a 2 stop difference.
In the third example, the distances are larger, but the ratios are the same as example 1.
How this applies to your question, if you blasted a color gel on the background by itself, you could get a good color. Add to that the main/fill, it then washes out because the main/fill are white light. Keeping the intensity of the main/ fill off the BG will keep color saturation.
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