The response/QE for a sensor isn't linear across the whole visual spectrum, so you need to calibrate and weight accordingly. For example, with my little Sony sensor, it's more sensitive in blue (1st) then Green (2nd, but close to blue) then Red.
I've been experimenting with calibration strategies...the first was to capture short images of a G2V star....I racked it out of focus a bit so it was easier to grab, and then calculated the average value over a 100x100 square of pixels from the middle. For better or worse
Knowing the ratios of response means you could also expose for different lengths to get you the 1:1:1 ratio. With my sensor, if I expose blue and green for 10 minutes each, then red looks about right when exposed for 12 minutes.