Color gel is a translucent or transparent coloured material that is used in theatre to colour light. Modern gels are usually made of polycarbonate or polyester, but the name arose from the gelatin originally used. See gel (theater).
Gels are also used in photography and cinematography for the purpose of color correction , and in this capacity are usually called color filters .
There are 3 main manufactures for theater gels. They are Lee (http://www.leefilters.com/) GAM (http://www.gamonline.com/) and Rosco (http://www.rosco.com/).
Similar colors may vary between different companies' formulations - for instance, they all have a color named 'bastard amber', yet Rosco's bastard amber is different from Lee's is different from GAM's. The range of gels includes non-colored media as well, such as a variety of diffusion and directional 'silk' products.
An important note is that the gel colors are not referred to by names (like bastard amber) but are instead referred to by a letter and number combination. For instance, G841 is a dark blue made by GAM, and R02 is a light amber made by Rosco.
All three manufacturer produce "swatch books", which contain a small piece of each color made by that manufacturer. Swatch books enable designers and technicians to have a true representation of the manufacturers' range of color.