At low temperature, Nitrogen dioxide, a reddish brown gas gets associated to form the colourless dinitrogen tetroxide as in the reaction 2NO_2(g) $\stackrel{ }{\rightleftharpoons }$ N₂O_4(g). Then at equilibrium

The intensity of color in the chemical reaction indicates the concentration of either reactant or product. Thus when the color has attained constant intensity, the concentrations of reactants and products have reached equilibrium values. In other words, an equilibrium is attained.