Lighting serves three main purposes on stage: to ensure visibility for actors and audiences, to establish mood through different colors and intensities, and to establish the time of day or era through simulated light levels. Visibility is important so audiences can see what happens on stage, while varying colors like orange, red, and blue can convey feelings like warmth, love, anger, and tiredness. Lighting designers also manipulate intensities and colors to simulate times like dusk, daytime, and nighttime to signify changes in scenes or time periods within a performance.