When my pikachu evolved from pichu, it’s electric sacks on its cheeks stayed pink instead of turning red. I’ve taken him to the Pokémon center and he has a clean bill of health. Is this occurrence common?

Absolutely. While all pokémon have a common coloration, an individual specimen’s coat, pelt, or so forth can actually vary wildly from the standard. And that’s not even referring to “shiny” pokémon or albino or melanistic ones. The variation can be as minor as a pikachu’s cheeks being the wrong shade.

So yes, anonymous, what your pikachu is displaying is actually a common occurrence, albeit not quite as common of one as the completely standard red-cheeked pikachu.