Paul, thanks - I really like StarTools' scientific colour method.
Re: G2V calibration you are right, it means finding a sun-like star (class G, stage 2V) which to our eyes emits pure white light. Against this, one can calibrate the R, G and B filters. To do this, I located a G2V star at the zenith, and took a number of exposures (of a few different durations) through each filter, and measured the ADU of the unsaturated star core. From that, I was able to work out the relative transmissivity of my R, G and B filters, and adjust the exposure duration of my subs to match this ratio. This will always hold thereafter for my filter/sensor combination.
More here:
http://starizona.com/acb/ccd/advtheorycolor.aspx