Here’s a good example of when to use a CTO gel. CTO stands for Color Temperature Orange, and it’s nothing more than a sheet of orange-colored plastic that you tape over your flash head to make the bright white daylight flash more incandescent colored. This top picture was taken on a tripod with the headlight on. An off-camera speedlite provided a little highlight in the paint and around the edges of the headlight. But, you’ll notice that the warm light from the headlight looks a lot more orange than the cooler light from the flash, and we’d like both light sources to match.

The solution? A CTO gel, of course. I taped a sheet of this stuff over my flash head and set the camera’s white balance to Tungsten (it was on Auto for the first shot). The result is that the light from the flash matches the light from the headlight (both are now orangish), but the Tungsten white balance causes the camera to cool off the warm light, making everything nice and white.

CTO gels come in large sheets for a few dollars, and you just cut them to the size you need. They come in different strengths, which you can combine to suit your needs. If you bought a sheet of #205 Half-CTO, you could use one layer for a little bit of correction, or two layers for twice as much. I bought mine from here.