Well, not real colors of the sky - but you get the idea.
The dominant influence factor is the climate. Winter is the most polluted season because of thermal inversion and less rainfall. Spring in northern China suffers from sandstorm. Still, you can easily identify the effect of government intervention, such as the significant improvement in Taiyuan. And look how amazingly Beijing performed in 2008 August through September for the Olympics.
Dedicated to my endearing home city.