Our Temper Colors for Steel Chart Post remains very popular. So here is a chart of Heat Treat Colors for Steel.
These days pyrometers are affordable. But it is the mark of a craftsman to be able to tell temperature by eye, if only to validate the instrumentation (or suspect it!)
These colors were obtained from a 0.40 wt. % carbon, alloy steel, as seen through a furnace peep hole during average daylight conditions.
Art of the Craft