Question

how does an iron rod change its colour when gradually heated

Answer 1

Many objects act as a black body. Iron is one such element. The heat supplied to the iron rod is absorbed by the iron rod. this absorbed heat in turn leads to the vibration of the atoms and the electrons inside. And then due to the excitation of the electrons which jump to higher orbitals. As the heat increases the excitation also increases and the intensity of the light given out also increases. So gradually the iron turns red and then slowly the color changes with increase in intensity.