>>3520210860% of the tapir and 70-80% of the giraffe are the same colour or close enough; the dominant colour can't be the one they both share and has never ever been the case.
Charmander and squirtle share a small amount of the same colour but are decidedly orange and blue respectively. Mudkip has a small amount of orange but blue is the dominant colour.
It's about balance and distinction. The gens they've added white to starters have been balanced by including darkbrowns or blacks on another of the same trio.
It's simple marketing and design - they emphasised the importance of starter choice during the Pokémon day celebrations - they aren't going to follow that by releasing Grey-pir and Grey-affe in the same gen.
