Colour gel

Gels do two separate jobs. Correction gels match one fixture to another: CTO, meaning colour temperature orange, warms a daylight-balanced source to sit beside a tungsten bulb, and CTB does the opposite. Effect gels are the magenta or teal wash on the background wall behind half of YouTube. Every gel costs output, and the deeper the colour the more light it eats, so a heavily gelled fixture needs to be brighter or closer than you expect. Heat matters on tungsten lamps. RGB LED panels have absorbed most of this work for creators, since the colour is dialled in with nothing clipped on the front.