Colored plastic sheet placed in front of light — shifts color temperature or mood by filtering wavelengths. CTO/CTB for temperature correction, colors for creative looks.
Using a color gel quickly becomes routine — you simply slide it in front of the light, and you change the color temperature or add a creative accent. The thin plastic layer filters certain wavelengths of light and lets others pass through. This sounds simple, but it's the basis for professional lighting design on any set.
Practical Use of Color Gels: You'll most often need them for Kelvin correction. If you have to shoot with artificial light, but daylight is coming in through the window, your artificial light will appear orange-tinted — too warm. This is where CTB (Color Temperature Blue) comes into play: the blue gel converts your 3200 Kelvin bulb towards 5600 Kelvin, matching it to daylight. Conversely, if you only have daylight and want to work with warm light, you use CTO (Color Temperature Orange) — the yellowish-reddish variant. CTB and CTO are available in different strengths (Full, Half, Quarter), depending on the desired correction range.
Beyond correction, it gets creative. Colored gels — magentas, greens, ambers, deep reds — create mood. A green gel on a Fresnel over an interrogation scene feels more ominous than white light; a soft magenta can frame a female close-up differently than neutral light. You pay for this in post — colored gels always cost you light intensity because they let less light through. So, you have to work with higher wattage or get closer.
Practical tip: Gels break easily, especially at high temperatures or if you mount them too close to the light source. Use clips or magnets, not tape — it sticks. And always have spares on hand; nothing is more frustrating than having to replace a damaged gel mid-shoot. Diffusion gels (like Lee 216 or Rosco 3024) are another category — they spread the light without significantly altering the color.