Electrical specs are very strict.

They aren't just about electrical safety directly, but also about not potentially confusing people in the future.

If the cable was black, then someone may not have known that it was an earth cable, and might have cut it, which would leave your electrical installation unsafe.

Thats the reasoning why using the wrong color for a wire can make it illegal.

That's the thing though: the best choice for a ground wire is no color at all, just the raw copper wire.

The part of the wire that is connected to the grounding object has no sheathing, but the length of the cable is insulated. The old cable was the same way. The coating was black and looked identical to the other wires running through the wall. I assume the justification for the code is that it makes the ground wire visually distinct.

It was probably sheathed in black or some other color originally.

Because without a common language for what color wires carry load a simple oversight can be fatal.

Except a bare wire is always a ground.

I believe that either a bare wire or a green wire is allowed. However no other colors. That was the case ~6 years ago when I checked the NEC.

Bare is allowed depending.

It is only required to be "primarily green". So the common alternative of green with yellow stripe is also allowed.

You can reidentify black cable if it's large enough gauge (6 awg currently).

I was a little surprised to read the original comment too. I've never seen a sheathed ground wire used in WA state (on residential).

"What on earth for?"

rimshot

* * *

Bare ground is allowed in lots of cases. The requirement is that insulated ground wires must be primarily green (IE green with yellow stripe is okay), but this is actually not new.

They must be > 6 awg to pull black and use green tape on the ends.

See 250.119.

This has been the case for a very long time though, at least since 2007 NEC. I don't have handy copies around for before then.