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I was going to say they used Malachite for green and Azurite for blue but those minerals aren't exactly commonplace nor found in large quantities.

Well, you know some guy gets the stuff and analyzes it and describes it and writes it up in a journal and he gets the credit but that doesn't mean some yahoo years before didn't figure it all out.

It's possible they had some material containing Chrome but didn't know what it was though I doubt it because the first known Chrome Oxide wasn't produced until almost 1800 and that was in the lab....

Chromium wasn't "discovered" until around 1780 or 90 or that is when the guy who isolated it is given the credit for. (actually 2 guys).