What/When color gels do you use? And why? (vs shifting your WB)

A whole book can be written on this alone. Nevermind. My most used gel is a variation on the regular CTO gel (Color Temp Orange). A home brewed mixture of 3 different gels that fits better  the usual halogen lighting at many venues. My aim is 2900K°. I'm filtering a flash, obviously.

The No1 reason you will gel a flash (or any kind of lighting equipment) is to match the light you're producing with the already present ambient light. Mismatched lights cannot be fixed in post, by adjusting the WB, and if they can, it will be with great difficulty. Better shoot it right in the first place. Gelling the house lights is usually out of the question.

Second most used gel for flash is another mixture that allows me to match ONE variation of the many neon fixtures I might shoot under. The most prevalent neon type I've seen is CW (cool white).

There are gels specifically made for these, but with some experimenting, you might find a mixture of gels that will match the regular CW much better than an already made CW gel.

Easy: shoot a grey card under the ambient light. Than shoot the grey card with gelled flash only (usually lowering the ISO, and using a fast shutter speed). Compare on the LCD, then compare on the computer using the eye dropper when you get close to perfect. Takes many tries, but when you find the right combination, it will serve you well for years to come. A cheap booklet of gel samples is plenty sufficient in order to experiment, end even to use on a speedlight.

Beware: a powerful speedlight may stick a dark gel to the flash's lens if it is fired repeatedly, or otherwise abused. Better use a loop of gel in front of the flash's fresnel lens.

Then there are the "theatre gels", those miriads of colors that are used for effect on backgrounds and hairlights, kickers, etc...

Have fun.

--
Jean Bernier All photographs are only more or less credible illusions