Even if using a one flash light set-up you often have two sources available – your flash light and any ambient light. If the ambient light is based on daylight – ie. you’re outside or working with window light, no problem. Flash is designed to be colour matched to daylight so the two meld well.
But what if the ambient light is drawn mostly from tungsten lights or from, shock horror, ugly office-style flourescents? You need to make your flash the same colour, or ‘colour temperature’, as that ambient. This easily achieved to correctional gels. Tungsten gels are called ‘CTO’ and come in differing densities depending on the ‘orangeness’, of the light.
Most household bulbs are around 2500K and a 3/4 CTO gel is probably the best match. You’ll seldom get the match exactly right – you merely need to make the light sources look as if they exist in the same universe. With digital cameras, set the color balance for ‘tungsten’ and shoot as normal. For even greater accuracy use a ‘grey card’ and create a custom white balance (Google it to see how). If you shoot RAW on your camera rather than JPEG you have much more leeway when it comes to correcting your white balance after the event.
Fluorescent lamps are more problematic as their are so many iterations and types, all with a different colour cast. Every fluorescent tube transmits different colours depending on the age and composition of the tube. Generally, it transmits light in a greenish range. In the household tungsten bulbs are being quickly phased out in favour of ‘energy saver’ fluorescent bulbs, which ten to be less greenish than the flickering office lights that give office-based workers migraines and SAD. Household fluorescent bulbs balanced to daylight – sometimes known as ‘full spectrum’ bulbs – are becoming more popular and theoretically should mix with flashlight much like daylight.
Due to the variation in fluorescent light there isn’t a one-size-fits-all correction for fluorescent light, although if you’re dealing with typical ‘office light’ we can get our colour into the right ballpark. Rosco and Lee manufacture green gels designed to balance the light from these light sources.
If the light source is just plain awkward and no gel in your possession seems to do the trick, the next, and sometimes the best, option is too simply kill the ambient ligth altogther by secting a suitable high shutter speed. You lose the ability for ambient fill, but if you have more than one flashgun on you this can be replaced.