I recently found a Mimikyu with a faded grey Pikachu disguise. Is it possible for Mimikyu to be colorblind? From what I read in my Pokedex, it’s clear that Mimikyu take good care of their disguises and I can’t imagine something so drastic as discoloration could be overlooked otherwise.

It is indeed possible, anonymous, but it’s extremely rare. This is why monochrome mimikyu are considered “shiny,” even though their shininess isn’t technically a genetic trait. Consequently, though, this mimikyu had attempted to capture a pikachu’s colors, but to that mimikyu, those colors just happened to be various muted shades of gray.

You are, in other words, fantastically fortunate to have come across this particular mimikyu, but I wouldn’t suggest pointing out that they got their costume’s colors wrong.