At low temperature, Nitrogen dioxide, a reddish brown gas gets associated to form the colourless

dinitrogen tetroxide as in the reaction $2{{\mathrm{NO}}_2}_{\left(\mathrm{g}\right)}\rightleftharpoons {\mathrm{N}}_2{\mathrm{O}}_{4\left(\mathrm{g}\right).}$ Then at equilibrium

At low temperature (0°C) ${\mathrm{N}}_2{\mathrm{O}}_4$ is stable and exist as equilibrium as pure colourless substances. When both flasks attain the temp of water both the colour intensity in both flasks become identical and no change in colour.