Our Temper Colors for Steel Chart Post remains very popular. So here is a chart of Heat Treat Colors for Steel.

Heat treat colors for steel Fahrenheit and Celsius temperatures
Heat treat colors for steel by Temperature

 
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.
Temper Colors for Steel Chart
Art of the Craft
Knowledge Retention