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Method:
1.) Set your camera’s white balance to the color temperature of your flash, the average color temperature of nikon flashes are around *6000k. The closest I could get is 5900k on my D200.
2.) Once white balance has been set take a picture of your grey card as you normally would to adjust white balance.
3.) View the image of the grey card on your camera’s LCD, place gels on the back of your camera’s LCD until the image of the grey card loses its color cast.
4.) Gel your flashes with the opposite color of the gels that you used to remove the apparent color cast of you grey card on your LCD.
4.) Done
It is easier than it sounds, for instance if it takes you 1 full cut CTB to remove the color cast on the image of your grey card, then use 1 full cut CTO to gel your flashes and balance the color.
Thanks to David Hobby of strobist.com for the inspiration.
Note: this method gets you in the ballpark and from my tests it gets you pretty darn close. I don't guarantee anything, I am just relaying my findings. also Equipment used nikon d200, nikon 50mm f1.8, nikon sb24 + DIY grid.
Pic examples below. Notes adjusted 1 frame and stamped the rest with the same settings for all color corrected images.

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on “How to balance your flash’s color with ambient light”