Again, remembering back to high school, if you starved the Bunsen burner of air, the combustion process was incomplete and the flame burned sooty red or yellow and at a cooler temperature.
The yellow colour is due to incandescence of very fine soot particles that are produced in the flame.
This type of flame only burns at around 1,000 °C.
Depending on the lighting, you may have actually seen the soot rising from the flame.
What you didn't see was that incomplete combustion was also producing dangerous carbon monoxide.
When comparing different gases
, you will discover that they required different amounts of air for complete combustion.
To burn properly LPG (Propane) requires an air-to-gas volume ratio of about 24:1.
For Butane, it is about 31:1.
Natural gas (Methane) requires an air-to-gas volume ratio of about 10:1.
Incomplete combustion also results in hazardous carbon monoxide:
Gas + Oxygen = Water + Carbon Dioxide + Carbon Monoxide + Heat