Right, so, under normal conditions when you look at your propane burner flame it will be mostly blue, with yellow tips. The yellow is because the flame is using oxygen in its combustion process. Under normal conditions, the propane is fully broken up into water and carbon dioxide (H2O+CO2). If there is not sufficient oxygen, for instance if there is a lid partially covering the flame, then combustion might be incomplete, resulting in production of carbon monoxide in place of carbon dioxide. CO instead of CO2. Of course if there is way too little oxygen, the flame will simply go out. Then you have the problem of propane gas being let out into the room.
This is why it's a good idea to have a combination propane/CO detector unit, mounted near the floor because both of these gases are heavier than air.
The Dire Wolfess