how to get white balance on flash and background?

You could set up a white balance card, something like the Better Balance, shoot a frame of it, then do a Custom White Balance. There are other white balance items you can get, some go on the lense, or some you shoot a frame with it in then use that frame to adjust the others in Camera Raw. I like the get idea others are talking about. But if you are using the flash on ETTL for a bit of fill I'm not sure how the gel will effect the image.

jack

White cards and custom white balance will not solve this problem. Picture this - shoot two people, one with an orange light and one with a blue. No matter how you adjust the color balance, it's not going to look right.

You've got 3 choices. 1. Just live with the mixed sources. Just reading an article by Chuck Gardner on another site. His position is that if the forground looks normal and the background has an orange cast, the eye will accept that. Depending on the situation, this may be an option.

2 - gel the flash so that the color of the flash is close to the existing lighting. Then use a grey card, custom white balance or sort it out by eye in post, it's all good.

3 - Again from Chuck's post - shoot raw and process 2 copies of the file - one for each color source. Combine them and use masking to select the appropriate sections of each. I haven't tried this but seems like a very good idea - now I'm wondering why I didn't think of it?

sleibrand wrote in post #5957191
I haven't tried this but seems like a very good idea - now I'm wondering why I didn't think of it?

Probably because you didn't try; it might work okay in some cases, but it can be a hellish job if the ambient is also (partly) illuminating the subject.
Have fun masking that