 We have a mixture of NO2 gas and N2O4 gas compressed in a syringe. We're going to expand the gas quickly. The color initially gets lighter due to the decrease in the concentration. However, if we watch the gas for a moment, the color becomes darker as more NO2 gas forms. If you decrease the pressure on a gas's equilibrium, the equilibrium shifts to the side with more moles of gas. In this case to the side of NO2.