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That's a good question fsr, and sounds good in theory, however the formula
L=(R+G+B)/3 would only apply for a perfect CCD with a constant QE across the whole colour spectrum.
Most CCD's have a QE of 60% at 550nm(Green), dropping to 45% QE at 650nm(Red) and 35% QE at 400nm (Blue/Violet).
You would also need to take into account atmospheric extinction, where blue and green signals are lower than red when your telescope is pointing at a lower elevations. At 50º elevation the blue signal is 5% less than red, and green is 3% less than red.
So I think what you say would work, but to get a good colour balance your formula to get blue may have to be revised slightly.
Cheers
Bill
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