Yes, the color on the new Legacy is lighter than the TMCC GS2...for sure. Again, I don't have the older Legacy version so someone else will have to make those comparisons. We do already know there is also 2 different colors of Lionel passenger cars, so which one matches the new Legacy model? and hopefully the new 21" cars match the new GS4
By "2 different colors of Lionel passenger cars" I assume you're referring to the (1) Daylight streamlined aluminum 18" cars that came out at the same time as the TMCC GS-2 (and I think the heavyweights that came out at the same time as the first Legacy GS-4 are the same color), and (2) the Shasta Daylight 18" streamlined aluminum cars. I think it has been pretty well established that the Shasta cars have incorrect colors (they matched diesels that came out in the same catalog).
Looking at all the photos and video I could find, it appears that the first Legacy GS-4 has paint which is pretty much the same as the TMCC GS-2, which means it too has a darker shade of orange paint than the new Legacy GS-4. I think the Legacy GS-2s (introduced after the first Legacy GS-4) also had the same paint, so the new Legacy GS-4 seems to be the odd ball in the group, colorwise.
If this is true, the earlier aluminum streamlined (non-Shasta) cars won't be an exact match for the new Legacy GS-4. Presumably the new 21" cars will match the new engine, but it will be interesting to see if they actually do. Hopefully they will, as you mentioned, although that won't help people who have earlier engine versions who want the new 21" cars (and the earlier 18" cars are pretty scarce). Personally, the 21" cars are too big for my use, and I already have the 18" set, so the fact that the new GS-4 has a different paint shade is somewhat disappointing. I was waiting for this engine - now it looks like an earlier engine version will be in the cards.