hi stephen
i don't think you can blend them in registax. you will need to use something like photoshop to copy each finished stacked image to the appropriate channel.
GIMP is a free software which is really just as good as photoshop these days.
https://www.gimp.org/
I'm by no means a planetary expert but my workflow would be something like below:
1. Use PIPP
https://sites.google.com/site/astropipp/ to centre planet and align all frames - separately for R G B
2. Run Autostakkert to use lucky imaging and maybe apply a drizzle or sharpen;
https://www.autostakkert.com/
3. Registax open the image up and use the wavelet sharpening
4. Winjupos to apply derotation
http://jupos.privat.t-online.de/index.htm (also Winjupos can put rgb together you just need to have the naming of the files in winjupos format - PIPP can help with that too)
5. Photoshop to put the image together (if not done in winjupos) - applying to the right channel.