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When using fill flash to supplement ambient light, how do you determine the flash color that will match the ambient light?

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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:

  1. Stick CTO gel on flash
  2. Set color temperature to tungsten
  3. Shoot

This has two possible effects:

  • If ambient light is tungsten, everything will look just normal
  • If the ambient is normal daylight, you foreground will have proper color, and everything else will be toned to blue. This can provide nice color separation effect (example)

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.)

Full description, examples and links to much more can be found in article at Strobist, overview of various gels can be seen at Rosco.

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  • \$\begingroup\$ The Rosco link shows Full CTO as being 3200K, and 3/4 CTO as 3500K, might want to clarify your explanation. \$\endgroup\$
    – chills42
    Commented Jul 21, 2010 at 13:29
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    \$\begingroup\$ Tungsten indeed seems to be 3200K; fixed. You're right about 3/4 CTO, but I had quater CTO in mind, which really goes to 4600K, at least per Rosco. Anyway, the point is that there is really wide range of color correction gels available for tuning the light. Full gel set for small flashes can be bought for around $10. (Google for "strobist gel collection".) \$\endgroup\$
    – che
    Commented Jul 21, 2010 at 14:48
  • \$\begingroup\$ I found early on that hslogen lights are higher temperature then classic light bulbs, not matching tungston ballanced film. \$\endgroup\$
    – JDługosz
    Commented Dec 11, 2014 at 17:17
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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.

-Matt

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