A shapeshifter standing in front of a mirror for hours making subtle adjustments to their disguises like a gamer in the character creator.

Is this a Monster Factory reference?

Plenty of humans desperately want this

Chameleons don't actually change color the way people think they do.

First off, the natural state of a chameleon is green, which blends in to the forest fairly well. Their color change is a mating tactic, and so favors bright and vibrant colors.

A chameleon changing colors isn't hiding, they're horny.

Or angry. Or scared, or well, any strong emotion. Basically a chameleon is a living mood ring.

Now, if you want true instant camouflage, that's an octopus. They can not only change colors, but their own surface texture. In their case, they get the info about what to look like from their numerous suckers alone their legs.