\({\lambda _m}T = constant\)

\({\lambda _m} = \frac{{constant}}{T}\)

So when a piece of iron is heated, \({\lambda _m}\) decreases \(i.e.,\) with rise in temperature the maximum intensity of radiation emitted gets shifted towards the shorter wavelengths. So the colour of the heated object will change that of longer wavelength \((red)\) to that of shorter \((reddish\,yellow)\) and when the temperature is sufficiently high and all wavelengths are emitted, the colour will become white.