The "heater" in your car does not actually generate heat itself. It actually takes the heat generated by the engine and passes that back through your cabin rather than venting it outside. So neither heaters nor winter will increase CO2 emissions.
Californians use approximately 1.8 billion liters (472 million gallons) of fuel each year running car air conditioners (do note that this is of 76 billion liters, or 20 billion gallons total statewide - so air conditioners account for about 2.5% of fuel consumption).
The reason that lighter car colours could be used to reduce air conditioning use is that they reflect more incoming light rather than absorbing it and heating up. A black car can heat up by almost 6°C more than a white car.
So, yes, a lighter coloured car would reduce the need for air conditioning, lowering CO2 emissions.