Is the green grounding screw chemically treated green for better conductivity or simple painted green?

Easy identification. It is a self tapping screw.

Now we are getting down to nuts and bolts.

As Don mentions, it is identifed with a permenent green color to indicate grounding terminal, which is the dedicated color code for grounding by the National Electric Code.

It is plated with zinc chromate tinted with green pigment.

http://www.metallicsonline.com/catalog/page127.pdf

Everything You Need To Know About Zinc Chromate (http://www.colorserver.net/history/history-zinc-chromate.htm)