Since I've run across a few posts where the problem posted about can be solved by custom flash gels, I'm re-posting my tutorial on making your own custom flash gels to match your lighting to any other light source. You'll need the following things:
1. A printer that can print on transparency stock.
2. A supply of said transparency stock.
3. A camera that can shoot RAW files.
4. A properly-exposed RAW image of a good white reference (Color Checker, WhiBal, etc.) illuminated with your strobe or flash only--no ambient lighting. The white reference part of the image should be about 1/3-2/3 of a stop from clipping.
5. One of the following:
A. If you are matching an ambient light source, a properly exposed RAW image of your white reference illuminated only with the light source you want to match.
B. If you are matching a display such as an LCD or TV, a properly-exposed RAW of the display displaying white (RGB 255,255,255).
6. An accurate printer profile for printing on the transparency stock.
The first thing you'll need to do is shoot the RAWs (items 4 & 5). Open the RAW shot with flash (item 4), do a click white balance on the white reference, and make a note of the white balance settings--color temperature and tint. Then open the RAW shot with the lighting you're trying to match (item 5) and convert it with the white balance settings you wrote down while converting the flash RAW. Make sure you're in 16-bit mode.
Take a 5x5-pixel eyedropper color sample of the white reference portion of the image, which will of course not be white. Crop the image to the size of the gel you want to make with a low resolution setting, like 1 pixel per inch or centimeter. Using the paint bucket or brush tool, fill the entire image with the sampled color.
In Photoshop, convert (NOT assign) the image to the transparency stock print profile. Then do Image / Adjustments / Exposure, and adjust the entire image so that the highest adjusted RGB channel value is between 245 and 250 (e.g. 229,249, 215). This will ensure that the printed gel is just dense enough to do its job, but is no denser than it needs to be. Now all you need to do is print this file on the transparency stock with the proper settings, and you have a custom flash gel that will perfectly match the flash or strobe to whatever ambient lighting you chose.
You can adapt this technique to make gels to perfectly match several flavors of lighting to each other, or to match your strobes to the color characteristics of a TV or monitor. Enjoy!
