Even if your filters are 1:1:1 combine in a laboratory, that doesn't mean you don't need background subtraction or slightly different ratios in practice...
There's no such thing as a purely black sky background. At a minimum, you're shooting through
airglow, atmospheric conditions can pick up and scatter light pollution from hundreds of kilometres away, and shooting through the atmosphere will cause extinction effects (especially for the blue channel).
Have you tried G2V calibration (e.g. see
here)? eXcalibrator seems to do a pretty good job of figuring out combine ratios empirically, but you'll still need to subtract background gradients (even at a dark site).
http://bf-astro.com/eXcalibrator/excalibrator.htm
Good luck
