When using fill flash to supplement ambient light, how do you determine the flash color that will match the ambient light?
That comes down to color temperature of the ambient light. Flash always has something similar to daylight (5500-6500K), so you need to use conversion gels from daylight.
Most useful gel is CTO (color temperature orange), which will color daylight to tungsten (3200K). Usage is as follows:
This has two possible effects:
Other usual gel is window green, that converts daylight (e.g. flash) to fluorescent-like green. Usage is similar to full CTO.
People also use half- and quater CTO, which convert daylight to 3800K and 4600K. These can be used for less-visible separation, or to warm up light for portraits. (Usual scenario: light some stuff with ungelled flash and the person with 1/4 or 1/2 CTO-gelled flash; an example with a bit more complicated setup can be seen here.)
If you're using flash as fill light outdoors on a sunny day you shouldn't need anything.
If you want to get a bit funky though, on a bland day with a grey or white sky, you can gel the flash with a CTO to warm up your subject, then set the WB on you camera to cool the shot back down again. This has the effect of lighting you subject correctly while having a cool blue sky that was never there.