According to Wien's displacement law, the wavelength $\lambda_m$ corresponding to the maximum spectral emissive power is inversely proportional to the absolute temperature $T$ of the emitting body. Mathematically, it is expressed as $\lambda_m T = b$. As the temperature of the iron increases upon heating, the wavelength corresponding to maximum emission decreases. Therefore, the color of the heated iron shifts from a higher wavelength (dull red) to lower wavelengths (reddish yellow), and eventually it emits all visible wavelengths, making it appear white hot.