At low temperature, Nitrogen dioxide, a reddish brown gas gets associated to form the colourless dinitrogen tetroxide as in the reaction . Then at equilibirium

Colour intensity remains unchanged with time.At high temperature the bulb glows reddish brown because backward reaction is favored.At low temperature the bulb becomes colourless because forward reaction is favoured.At equilibrium state a faded reddish brown colour is observed which remains unchanged with time