The trouble with heating to achieve a certain color is that the surface
is not protected. The part WILL rust as soon as the humidity goes up.
Years ago, I took a basic blacksmithing course and made a half
dozen hammers, fullers, etc.. The were hardened, polished, then
tempered (the process you are talking about) from the faces inward
toward the eye so they showed the full range of temper colors. They
looked great for a year or two in my shop, but the first time I took
them out for a weekend of demonstrating to the public, it rained and the
humidity in the air rusted everything and my pretty colors were all gone.

Pete Stanaitis
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it is called heat.

clean the surface. sit the item on a square of copper sheet for even
heat dissipation and heat the underside. when the item shows a blue
colour all over tip it into the oil to quench.

the trick is to be subtle with the heat.

Stealth pilot