Actually, that's the thing--Pichu and Plusle/Minun
don't share the same main body coloration, which is part of the reason I supported the idea of Pachirisu evolving into Emonga at first:
As you can see, while they have kind of similar colorations, Pichu is yellow, whereas Plusle/Minun are both more a cream color. Pachirisu and Emonga do both have the same main body color of white though (and they both also don't have any sort of markings or anything on their stomachs--hey're just pure white), in addition to having the same color cheeks and Emonga having two triangular stripes on its forehead just like Pachirisu's singular one. Of course, due to stuff like Emonga being in the demo, I don't actually think it'll turn out to be a Pachirisu evolution, but were that not the case, it's definitely quite easy to link the two, as they do share enough similarities (moreso than other non-related Pokemon, and are more similar than Pikachu and Raichu are in terms of coloration and such and interestingly, Pikachu's tail is also one of the things that really changes about it when it evolves) to suggest some sort of relationship.