Transparent colored gels slip neatly in front of traditional theater lights, known as parabolic lights or par cans. Reds and yellows create a sunrise; purples and pinks indicate sunset. Use blue in night scenes, especially those set in cemeteries or haunted houses. Adding a fog machine makes a scary situation far more frightening. Filtering red lights through the same fog machine imitates smoke and fire, from a small house fire to the burning of Atlanta.
Cut tin sheets into different shapes and slide them into the par cans to cast silhouettes against a backdrop. Think of the bat signal and you get the idea. Called gobos, these images can be everything from black cats to birds in the sky or a full moon.
Black lights against white clothes makes ghosts, angels and other spirits shine.
Shine light through a glass bowl filled with water for a seafaring, poolside or fountain shimmer.
White Christmas lights above the stage build a starry sky, or if under fake snow, a winter wonderland. Putting them in a backdrop window gives the impression that the lights are on. Red bulbs in a fake fireplace make it glow.
Experiment with light intensity, from soft glows to sharp whites, to create different atmospheres. Try different light angles to throw shadows on the stage that fit the mood the director wants.