I believe I would call it "heat treatment" or "drawing" or "tempering".
The colors range from light yellow through red and blue to grey. It can be done for decoration but is more often the result of drawing hardened parts to render them less brittle.
Fire blueing is a commonly used mis-nomer, there is a "fire" blue , sometimes called carbon bluing or carbonia bluing; I believe Colt was "famous" for this very pretty but not very durable blue.
Here are the colors and temperatures required to achieve them on polished steel. These are not "my" charts but borrowed from other posts on these forums.
