Colour gel / Gel filter
Gelfilter · Light
A coloured transparent sheet placed over a flash or lamp to change the colour temperature or colour of the light.
Gels are used for two purposes: correction (an orange CTO gel warms flash light to match ambient light) and creation (coloured gels for dramatic colour effects on the background or subject). CTO and CTB are the most commonly used correction gels.
Tools that help with this: