Meaning of temper color

tem'per col"or

Pronunciation: [key]
— Metall. Metall.
  1. any of the colors appearing on the surface of clean, unoxidized steel heated in air, from pale yellow at the coolest to dark blue at the hottest: used as an approximate indication of temperature.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.