I just found some great new freeware
here . It computes the RGB ratios you need to use to combine frames and is an alternative to G2v calibration. The manual is pretty clear and it should take under an hour to get familiar with what's going on.The only hitch you may encounter is that you need to plate solve the red image, so you need something like CCDSoft, TheSky or MaximDL. It uses catalogued data to identify star colours and computes an RGB ratio.
Geoff