Sometimes I get a NG message (it's been a while, I think that's what it says) on the LCD of the camera when I try to do a strong white balance indicating that it can't correct that much. It may be that whatever camera Gary is using has more latitude for correction in it's white balance settings than the K5. But you don't have to do it in camera, I shoot in RAW and normally do it in post.
Here's a shot that I did a few weeks ago. I don't think I took the time to set the white balance on location. Lightroom tells me that I used 7150K as my color temp and -26 (green) as my tint in the original. The shot was lit with a Full CTB gel on my Metz 58 AF2. I used high speed sync to underexpose the sky by two stops, my camera settings were ISO 200, 1/1500th @ f/5.6. This was taken using my Sigma 8-16 @ 14mm. Now normally that's not a portrait lens, but it allowed me to get very close to my subjects, which was important because in HSS mode, the flash power is greatly reduced, and the blue gel made it that much worse. To be honest, my subjects were a bit underexposed in the original, but nothing that a little fill adjustment couldn't fix. For the final version, I used the eyedropper tool on a separate frame I shot of my gray card lit with the same flash/gel combination to get my final white balance of 50000K and +23 (magenta).
The keys are:
- Shoot in RAW
- Balance from a gray card in post production
- Underexpose the background and let the flash do the work of correctly exposing your subject
- Get close to your subject with the flash