White balance in mixed lighting
Back in the days of film, colour temperature was adjusted using correction filters. Filters are still used to correct colour in digital photography when White Balance adjustments aren’t enough. This happens if you’re shooting with two different light sources.
The White Balance can’t be adjusted for, say, flash and ambient light, and if you adjust it to match the flash, you’ll get a yellow-orange background, as above.
By putting an orange filter (known as a gel) over the front of the flash, the colour of both light sources can be made similar, so you can choose a White Balance setting that matches both.
How to set up your new Nikon DSLR
How to make a preset manual white balance setting on your Nikon DSLR
White Balance: how to use a colour chart to get tones perfect
What is white balance: common problems and how to solve them