I recently helped a friend upgrade his service in a Chicago suburb that follows Chicago code. We failed the inspection because the inspector said that the bonding jumper from the service entrance conduit to the neutral/ground bar was green and the code specifies it "shall not be green or white". Is this true? He suggested black wire with no green tape? Also the GEC to the ground rod was green and he said that was no good but he didn't write it up.

Only in Chicago.
I wonder what that is all about? What color is Bare?

Copper color?

Which becomes green after a few years of being outside. Do I need to clean my GEC annually to prevent tarnishing if I choose a bare conductor?

Well, it is correct that the NEC does not allow the wire from the panel to the grounding electrode to be green, or white.

The only way a green color on the GEC is non-compliant is if it is a color on the insulation. 200.6 defines the colors of insulated wires.

That's just one of the challenges of working in Chicago.

You never know if a requirement is actually in the Code, a specific interpretation, a leftover rule from days gone by, or just that particular Inspector having a better way to skin the cat (or having it out for you).

It would be really nice if they had to give you specific Code references when they make their comments, but that'll never happen.