The above chart shows the 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. percent carbon, alloy steel, as seen through a furnace peep hole during average daylight conditions.
All Craftsman’s Cribsheets are available for viewing and download at short.productionmachining.com/cribsheets.