The colour of a piece of steel may be used as a guide to it's temperature. These are particulalrly useful when hardening or tempering steel.
Check out the generally accepted colors/temps for metals glowing red.
When it comes to copper, it will turn to dark red at temperature range 700°F-1200°F (371°C-649°C). It melts at 1981°F (1084°C).
The process whereby a metal is heated to a specific temperature /color and then allowed to cool slowly is called annealing.
This softens the metal which means it can be cut and shaped more easily. Mild steel, is heated to a red heat and allowed to cool slowly. However, metals such as aluminum will melt if heated for too long.
Scroll through the pages below to vie annealing temperatures of different metals: