Two brown nitrogen dioxide molecules join into one colourless dinitrogen tetroxide. The colour tells you which way it has gone, which is why textbooks and demonstrations use it so often.
The forward reaction is exothermic and the number of molecules drops from two to one. Cooling it and squeezing it both push towards the colourless side.
Warming it brings the brown back. Compare the mix of colours among the particles with the two lines on the graph: they say the same thing.