When you use paint, it reflects the colors of light that you are using, and all others are absorbed in to the material. With light, you are actually using colored light, and that mixes things up. Red, Blue, and GREEN are the primary colors of light. Cyan, Magenta, and Yellow are the secondary colors of light. When you mix blue and green light, you get cyan; red and blue, magenta; red and green, yellow. The hardest color to make with light is yellow. While all of the primary colors of paint can be mixed together to make muddy mess, the primary colors of light can be mixed together to make white.
Physics lesson time:
So, when you paint, and you mix colors, and you reflect the colors of light that you paint (as I have already said). A lighting
gel takes out all of the colors except the specific color of the
gel. Then, when you mix the colors of the different gels, both colors of light are reflected in to your eye, activating different cones in you eye. This creates one color of the nerve signal sent out of you eye. The reason that this isn't the same as paint is that with paint, you are actually mixing the pigments together to reflect one color of light.
The light mixing color wheel:
http://upload.wikimedia.org/wikiped...ColorMixing.png/145px-AdditiveColorMixing.png
Experiment with primary and secondary color mixing here:
http://www.wackerart.de/mixer.html