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(Some of) this thread seems to be the question 'What colour is grey' or white, or black. Can we agree to leave aside inks and paints, and talk light (either transmitted from a CRT or filtered through an LCD system).
If by colour (or color) we mean chrominance, in the YUV world that we all work in, then black through to white have zero chrominance. Just a Y luminance signal. True for analog or digital.
But if we're talking RGB signals (like we used to have out of studio cameras) then black through to white have equal values of R G and B** so they've got all the colors.
So everyone's right. And to address the original question, if you're trying to key on colour difference, white's a non-starter. If you're keying on luma level - best of luck, hopefully the talent is not very bright :-)
Philip Ashby
Bright Filament | Bath UK
**Just to forestall the other engineers amongst you, I'm choosing to ignore the different weightings of the colour signals in the interest of concision here. (In explicit terms, Y = .3R + .59G + .11B, on this side of the pond anyway. In the manner of Mary Tudor and Calais, it's etched in my heart, like the value of NTSC and PAL subcarriers. 4.43361875 +/- 1 MHz since you ask)
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