ok. check this out.
All steel discolors as it is heated, and slightly different types of steel will color slightly differently, depending on their alloy. However, all of them follow a predictable progression. If steel is heated to the temeratures shown below and then cooled, it will take on the color shown
Here are the colors and temps for standard mild steel. Yea, I know headers are stainless, so the progression is a bit different, but you get the idea...
Colors for Tempering Mild Steel
Very pale yellow 430 degrees F 221.1 Degrees C
Light yellow 440 226.7
Pale straw-yellow 450 232.2
Straw-yellow 460 237.8
Deep straw-yellow 470 243.3
Dark yellow 480 248.9
Yellow-brown 490 254.4
Brown-yellow 500 260.0
Spotted red-brown 510 265.6
Brown-purple 520 271.1
Light purple 530 276.7
Full purple 540 282.2
Dark purple 550 287.8
Full blue 560 293.3
Dark blue 570 298.9
Very dark blue 600 315.6
Before digital thermometers, we used to use these colors to know how high we had heated a piece for heat-treating purposes.
