Just a little snap of some steel brackets I made for a house on Guemes Island- I ground the crud off of each one so that paint would bond better, then when the backsides got welded there were some nice heat colors.
These colors are used for judging heat when tempering cutting tools, also. Tool steels are at their hardest at a very light brown, striking tools reach a good compromise of hardness and toughness in the purple tones.