Originally Posted by Murray@uptowngallery
They will obviously work for filtering flash. If you're going for really saturated colors/shadows, you'll probably end up wanting something like Lee or Rosco theater gels with stronger or more saturated colors than CC or CP filters. My guess is that the effect will be a little subtle unless you're just trying to color balance the flash.
I'm not surprised that high quality CP filters perform well under the lens for Ralph and others. I've seen some that wouldn't do well in that application, especially when stacked up 4-6 filters deep as some color printing might require. Unless they're Microsoft or Apple brand filters, you're probably not even violating the EULA by using them under the lens or in front of a flash.