Why the lpg gas burn with blue flame and after ending point it burn with yellow when it goes to empty?
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Answer:
You generally get a blue flame from burning wood as well, it's just usually overwhelmed by the yellow, and hard or impossible to see. Try looking at the bottom of a match just before it goes out, and you'll usually see the blue.
The blue is called "chemoluminescence". It's the light produced directly by the combustion reaction, and will therefore be present during most hydrocarbon fires. The yellow and orange (which provide most of the light from the actual fire) come from a more interesting source. When hydrocarbons burn without sufficient oxygen mixed in, they burn incompletely. This reaction produces tiny particles of solid carbon, which we generally refer to as 'soot'. The flame is full of this finely dispersed carbon, which gets heated by the combustion. A typical wood fire produces flames anywhere from 1000 K to 2000 K. At that temperature, the particles glow, giving off orange to yellow light. Those tiny bits of superhot carbon is what causes most of the light we see from fires.
Gas burners are generally designed to mix plenty of air in with the gas before it burns. so that incomplete combustion doesn't happen, soot doesn't get produced, and so all you see is the blue from the combustion. But if you block some of the air from getting into the burners, you'll find that gas flames burn just as yellow as wood.