A piece of iron is heated in a flame. It first becomes dull red, then reddish yellow, and finally turns white hot. The correct explanation for this observation is possible using:

According to Wien's Displacement Law:
$${\lambda }_{max}T=\text{constant}⟹{\lambda }_{max}\propto \frac{1}{T}$$
As temperature $T$ increases, the wavelength ${\lambda }_{max}$ corresponding to peak intensity decreases. Since red has the longest wavelength in the visible spectrum and blue/white have shorter wavelengths, the color shifts from red to yellow to white as the iron gets hotter.