Gel before light or lens — shifts color temperature instantly. Standard: CTB (3200K→5600K) or CTO (5600K→3200K) when mixing sources.
On set, you need color conversion filters when you have to combine light sources of different color temperatures—and this is constantly the case. You have daylight from outside (5600K) but want to illuminate with halogen spotlights (3200K), or vice versa. Without filters, you get color chaos: one side of the face is orange, the other bluish. The solution: you place a filter sheet in front of the light or in front of the camera and bring everything to one Kelvin level.
The standard filters are CTB (Color Temperature Blue) and CTO (Color Temperature Orange). CTB converts warm tungsten light (3200K) to the cool daylight spectrum (5600K)—you place the blue sheet in front of your halogen light, and suddenly it matches the window lighting. CTO does the opposite: it takes the harsh daylight and warms it down to artificial light temperature. You need both types in your kit because the situation on set determines which way you go.
Practically, it works like this: The filters are thin sheets—Rosco, Lee, Gels are the standard manufacturers—and you attach them in front of the light or in front of the camera lens. With lights, it's common to attach the filters directly to the reflector because you have more surface area there and less light loss. With the lens (as a front-mounted filter), you use them more in emergencies or for very specific corrections. Note: CTB filters absorb significantly more light than CTO—a full CTB conversion costs you about 1.5 to 2 stops of light. So, you have to set your exposure higher or use a longer shutter speed when using CTB.
There are also partial filters—half conversion filters (Half CTB, Half CTO)—for more subtle adjustments when you don't want to switch completely. Some DoPs also use them to balance mixed lighting situations: you light one side warm, the other cold, and both lights work together instead of against each other. Correcting the white balance in the camera or in post-production is usually more complex and creates color casts—it's better to get it right on set. So, color conversion filters aren't fancy, but a practical necessity. Without them, you'll be working with artificial light in daylight rooms and creating optical conflicts that no color grading can fix later.