Thanks Tom, I've appreciated your thoughtful and detailed explanations on what I've been attempting to do with the images I've been getting (and I suspect that you are probably getting a little tired of telling me that it's not possible ). The idea behind the "Photometric Color Calibration" sounds a bit like what I was trying to do with my G2V star calibration method a few months ago, which was to adjust the RGB values to be more-or-less equal for the Sun-like star. The analysis showed (or at least didn't contradict) that my method of using Wred=62, Wblue=99 and multiplying the blue channel in Registax by 1.1 produced the right colours for both the planets and the G2V calibration star. Now, there may have been more than a little confirmation bias going on in that analysis, but the results were at least consistent. Of course the planets are being illuminated by the Sun so they should be in "daylight", unlike the stars which have their own spectral characteristics.
The colour balance of my Jupiter images match pretty closely the "Auto-balance" RGB settings in Registax, and they tend to match other's images of Jupiter also. Probably because of the wide colour variations and large white areas on Jupiter, Registax is able to provide a good auto-correction of the colour channels, something that is not present in other planets (especially Mars, Uranus and Neptune). This all started for me when I started taking images of the Ice Giants and wondering, how do I white balance these planets when there's nothing to balance against?
My key question regarding this is: If I take images using the same colour settings and with the planet at the same elevation, why should the resulting colour balance be different?
The only thing I can think of to answer this question is that somehow either the stacking process or the sharpening process introduce an unequal effect for the different colour channels. For instance, if the Red channel is higher than the Blue in the original image, does the sharpening push the red higher and blue lower, thus increasing the imbalance? This is something I plan to look into next.
I do use Registax's Colour Balance tool for looking at the overall colours of the planets, but of course the Auto-balance feature is not applicable for Mars, Neptune or Uranus (and I contend Saturn also) which is why I'm interested in a standard method. I could use the icy polar region for auto-white balancing Mars in PS, but I want to have something that is consistent.
I was especially interested when Christophe posted his spectroscopic measurements of Uranus and Neptune, as these are the most definitive measurements of the "correct" colour of the planets I can imagine, the only problem is trying to then incorporate the effects of Earth's atmosphere and how that affects the overall colour balance. This is not insurmountable, but still something to consider.