One of the toughest concepts I've had to learn was HOW to use gels to match/complement ambient light. After a lot of trial and error, I've finally realized the magic: it's two parts.


Part 1: Correct for color temperature, i.e. the red/blue shift. I use a gray card in Kelvin white balance, and tweak the Kelvin value until red and blue are equal on the RGB histogram (or a little red for warmth...this is still elusive for me). I use this color temperature reading to know/guess which level of CTO to apply to my flashes.


Part 2: Correct for green shift, i.e. the balance of green vs. red/blue. Still using the gray card, I usually end up dialing in some minus-green to get the green value to align with the red/blue values. I use this green shift value to know/guess which level of plus-green to apply to my flashes.


I think my confusion came from Strobist 101, where he mentions that you'll use both orange and green gels very rarely. I find that fluorescent lights are often 4000K AND greenish, so I have to gel orange to convert flash 5600K to 4000K and gel green to match the fluorescents' awful green tint. I'm working on a chart to help me determine which CTO and which green to use, still a work-in-progress.