According to PipeDown :

As for color, Code allows for non standard colors to be used as long as the
conductor is identified by the proper color tape at each end. Not a great
idea IMO if you can avoid it. For example the SE wire (for a 200A service)
only came in black, I had to tape off the neutral with white electrical tape
to identify it. The inspector and power company guy had no problem with
that.

There's a specific exemption for service conductors. It's more complicated
with circuit conductors. You can mark a white conductor black to indicate
it's hot. But you can't do the reverse.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.