1 ACCEPTED SOLUTION
You can't white balance stage lights because it's not just one color temperature of light.
If you're shooting a scene which is light entirely by the same type of florescent light, then you can use white balance to correct for that. But if you're shooting a scene which has some "white" incandescent lights and florescent lights in the same picture, then you can correctly white balance only for one of those light sources, but not the other (not both at the same time.)
Stage lighting is a bit like this because there are often red, green, and blue floods. Ideally the point of these was that for theatrical lighting, they could "white balance" the whole scene because the lights where all aimed in the same place. So by slightly turning up the red and turning down the blue, they could "warm" the scene. I've noticed that in some venues, they don't converge the lights (the reds, blues, and greens are not necessarily even pointing in the same place.) You'll need to do your light adjustment artistically by what you like as there is no "correct" (since the lighting was not set up to create "white" light nor anything even remotely close.)
Take a look at the image below:
Look carefully at the keyboard. We know all the keys are the same "white" color in real life. But the keys at one end of the keyboard have a blue/green color cast while the keys at the opposite end have a warmer orange cast. You can see the same effects if you look at the collar area of his white shirt. In the end, I look at the skin tones on his face and do the best I can with it.
If you were NOT shooting stage lighting, then yes... a gray card will help you render more accurate color temperatures, but not necessarily more accurate color saturation levels. A color card (e.g. something like an X-Rite Color-Checker **bleep** card) will help you get the saturation levels correct.
The human eye is mostly sensitive to greens because that's smack in the middle of the visible light spectrum. We are less sensitive to violets and blues (especially violets) and also less sensitive to reds. A digital sensor with no bayer mask is technically somewhat equally sensitive to wavelengths across the range, but the camera has a a "bayer mask" overlaying the sensor which is has more green than anything and on top of that, the filter in front of the sensor trims light transmission for the wavelengths on either side of the middle of the spectrum. This helps the camera more closely approximate the sensitivity of the human eye so the photos look more "natural" to us humans (despite the fact that our eyes don't perceive all wavelengths of visible light equally).
However, what the camera does just the best approximation of the human eye (and we actually do know that different humans have varying levels of sensitivity ... you might actually see more violet than I can see. Also this changes with age. The amount of violet you saw when you were 10 was probably stronger than the amount of violet you can see when you're 50.) A camera, might be a little more sensitive to, say, reds than it is to blues. The idea behind the color checker tools is to provide various squares of colors to get the color values accurate (not just the white balance.)
