The practice of using colours to determine the temperature of a piece of (usually) ferrous metal comes from blacksmithing. Long before thermometers were widely available it was necessary to know what state the metal was in for heat treating it and the only way to do this was to heat it up to a colour which was known to be best for the work.

The colours which can be observed in steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

are:

Colour	Temperature [°C]
Black red (when viewed in dull light)	426 to 593
Very dark red	593 to 704
Dark red	704 to 814
Cherry red (lower critical point)	815 to 870 (critical point depends on carbon content of steel)
Light cherry red	871 to 981
Orange	981 to 1092
Yellow	1093 to 1258
Yellow white	1259 to 1314
White	1315+