Wow so OK amends the code and mandates that color coding huh?"What code, if any, requires different phases to be different colors?"
In most case the local AHJ has some sort of amendment that states color coding rules.
I bring up AHJ due the fact the way you ask your question.
I see they still mandate 'natural gray', I have noticed some wire manufacturers no longer offer 'light gray' or natural gray.
But I sill ask for it, I dont like that dark gray.