Reaction of colorless dinitrogen tetroxide (N2O4) to form brown nitrogen dioxide (NO2). Initially, only N2O4 is present and only the forward reaction (decomposition of N2O4 to give NO2) is occurring. As NO2 forms, the reverse reaction (recombination of NO2 to give N2O4) begins to occur. The brown color continues to intensify until the forward and reverse reactions occur at the same rate.