The combustion of gas produces a flame. which may be Blue, Red, and Yellow.
Combustion of gas with Red or Yellow flames is considered as bad combustion or incomplete combustion of gas. whereas blue flames are a sign of complete combustion of gas.
Gas flames are normally blue but sometimes they burn red or yellow. A yellow flame indicates incomplete combustion, which means you are wasting gas and money, also it may serious safety hazard.
With hydrocarbon flames, such as gas, the amount of oxygen supplied with the gas determines the rate of combustion, flame colour, and temperature.
Due to incomplete combustion (with yellow or Red flame) we are not getting much heat as in the case of complete combustion (with Blue flame),
Combustion of LPG gas with Blue flame produces approximately 1,980°C of heat.
Combustion of LPG gas with Yellow flame produces approximately 1,000 °C of heat.