From The Horse's Mouth

First of all, what we call gels are not gels at all. In the old days (when K.C. was starting to learn about lighting), color filters were made by mixing dyes (some were aniline, yikes!) with cow “by products” (rendered fat and hooves). This concoction was spread in a thin sheet to dry and then cut up into sheets. Nowadays, color filters are coated, deep dyed or color infused plastic based sheets.

A color filter does not “turn” the white light beam a color. A color filter does just that, it filters the unwanted colors out of the white beam of light which is composed of all colors (Roy G. Biv). The unwanted colors are retained in the filter in the form of heat. Eventually the filter will fade or burnout because of dye migration and is tossed out. People ask if there is a gel that will turn the light into a black light. For this to happen, the light source must produce a good amount of ultraviolet radiation. Fluorescent light, metal halide, and some incandescent lamps produce a high amount of UV light. Unfortunately, a Source Four with an HPL lamp does produce a bit of UV but not very much. If you can get the metal halide version of the Source Four, it will work much better. When we get into dichotic filters, it is a different technique altogether.

A dichroic does not absorb the unwanted wavelengths; it simply reflects them away from the main beam of light and allows the wanted color to proceed through. If you angle a dichroic filter in front of a light beam you will see what is called color fringing, which shows the reflection of the unwanted colors. The advantage to the dichroic is that by not absorbing the light, they remain cool and last considerably longer and are not affected by high UV rich light or heat. Dichroic colors are more saturated and available in more intense colors. Cost is considerably higher because of the manufacturing equipment and time it takes to produce them.