Ensure that you have a full assortment of functioning colored spotlights, including blues and reds. Make sure all bulbs are working by having your light tech bring up each light.
Read through the play you will be producing. Highlight any opportunities where the use of light might heighten the emotion of the scene. If you are unsure, just highlight any part of the script where a character expresses a strong emotion, such as sadness or anger.
During your tech rehearsal, have your actors run through the scenes you highlighted, testing different lighting effects. For example, if you are doing a production of Cinderella, you might want to cast a blue spotlight on the Fairy Godmother to give her a pure, magical presence. Play with various color combinations to see what creates the best effect.
Once you have decided on what color combinations to use for various scenes, mark these down on your script and ensure that your light tech marks them down as well.