Gels come in all colors, but the two types that we use most often are CTO (Color Temperature Orange) and CTB (Color Temperature Blue).
These aren’t any ordinary colored gels – they actually correct the light’s color temperature, making a tungsten light color balanced for day or an HMI lamp color balanced for night (night exteriors are usually lit with tungsten lamps).
The color temp gels come in full, half, quarter and eighth, which gives one the ability to decide just how corrected that light should be. Extra blue-y backlight in that night scene? Probably an uncorrected HMI standing in for moonlight.
This particular example is an HMI (balanced for daylight) with full CTO, which means that on film, it’ll look like a tungsten lamp.