Order a swatchbook of Lee and Roscolux lighting gels. These small books, similar to a paint samples, show the colors that each company provides. Select the shades that suit your production and that will benefit each scene. These books typically progress from amber to pink to red to shades of blue and green.
Decide the specific lighting needs of your show. If lighting a dance performance, keep in mind the use of side-lighting with saturate and darker colors in order to silhouette the performer with a colored outline. If designing for a musical, keep in mind the transitions from regular acting lights to a softly lit music number.
Assess your inventory of lighting instruments. This is a very important step in deciding how many periphery or "mood lights" you can use in addition to your basic illumination. Choose gels that allow the majority of the light through, such as amber-R02 and lavender-R51, if you have a minimal number of instruments. These will allow a good visual contrast without absorbing a great amount of the original light. Select darker, more visually striking colors, such as pink-R36 and blue-R68, if you have a lot of lights, so as to get the most out of your instruments. Generally, you should have at least three lights for every 6-by-6-foot area on stage.
Design your lighting scheme for the production. The popular Stanley McCandless theory mentioned in his book "A Theory of Lighting the Stage" supports using a key light, a fill light and a back light for each 6-by-6-foot "acting area" of the stage. Do this by placing one stage light with a warmer color at a 45-degree angle to the left of the stage, and another light with a cooler color at a 45-degree angle to the right. Choose your gels to provide a healthy contrast between the warm key light and the cool fill light, amber-R09 and lavender-R54 for instance. Next, place a third light behind the acting area as a back light, which helps visually distinguish the actor from the background. The most common choice for back light is blue-R68 and is particularly effective for establishing mood.
Make a checklist with all the desired gels and order your gel sheets from a theater and stage supplier. You can find these online. Cut the gel sheets to the size of your lighting instruments. Gel sheets typically come in 30-by-30-inch swatches, which should be cut to fit in the gel frame of your lights, typically 6-by-6 inches. Place the gels in the lights and test them on your actors, costumes and set pieces. Remember that gels often change color depending on what kind of lighting instrument you use. As most lighting instruments have halogen lamps, which generate a slight amber tone, your color will most likely alter somewhat at high intensity.