Both Shellder and Cloyster are purple they do not change. Charmander is orange and so is Charizard.
Charmander is orange, Charmeleon is a dark orangish red, back to Charizard who is orange. The color schemes change a little bit.
Squirtle is a lighter blue than both Wartortle and Blastoise.
Rattata is Purple and Raticate is brown.
Pikachu is Yellow and Raichu is Orange
Vulpix is a brownish Orange while Ninetails is yellow.
Psyduck is Yellow while Golduck is blue.
Goldeen is Pink and white, Seaking is mostly Orange
Staryu is brown while Starmie is purple.
Chikorita is Green, Bayleef is Yellow, Meganium is green.
Spinirak Green, Ariados red.
I'm going to stop here because the point is proven. You don't even have to match colors for it to be an evolution. So my earlier post was also wrong.
I'd be really happy with that Otter's 1st evolution, he looks way cool, same goes for Smugleaf. The Otter's 2nd evolution looks bad though, and Pokabu is just...well...bad.
Those two were in the Beta art but haven't been confirmed to be the actual forms yet.