I'm "ignoring" most of the changes in the UCS, but I have a suggestion: now we're using the RGB notation, what do you think if the Dominion and Sucessor state colours are padronized: for any given country, their Dominion is the original colour + 127/127/127/127 (hard grey with 50% of opacity), and the Sucessor State colour is 225/225/225/127(light grey with 50% of opacity).
I think this idea was proposed in the past, but not applied (other than to the french sucessor state and the british dominions...).
The result is clear, the maps are intuitive and, even if there can be some overlap, it simplifies quite a lot the colonial nations for ATLs - we can even keep the colours for the countries (the brazilian green/blue, the argentinean dark blue), but this would give a standard alternative for when an unusual country has colonies.
If a second colour is needed, we could use original colour + 0/0/0/100 (black, with some opacity).
Other than that, I agree with bright red as the colour for the main communist nation, rather than the Soviet Union specifically. I suggest, though, to use the same logic for the US colour: instead of being the british colony which broke violently, it should be the colour of the main independent ex-colony of America. It doesn't matter if it was founded by French, Italians or Morrocans - the major american power would receive the colour. This way, the map would give a little more explanation about the scenario.