燃燒爉燭時，黃色火光源來自邊道

The yellow frame is due to the oil in the candle.

To be more specific, that oil is the higher alkanes. The combustion (burning)
of the candle is incomplete, causing the flame becoming more sooty, and
therefore the flame is yellow.

Complete combustion will give out carbon dioxide and water. It is non-sooty so it gives out blue flame (e.g. town gas, which is the lower alkanes(C-1 - C-4).
However, incomplete combustion will give out carbon monoxide. Soot is
produced, resulting in giving sooty flame, which is orange in colour and yellow
(e.g. candle )(>C-25) shows that the flame is more sooty than the one given out
orange flame.

睇唔明嘅睇下面

黃色火光源的出現係因為燃燒爉燭時產生煙灰(因為燃燒作用是不完全的，完全的燃燒作用不會產生煙灰，所以是藍色的)。

簡單版
完全的燃燒作用：烷+氧氣---->二氧化碳+水，冇一氧化碳同埋煙灰。
不完全的燃燒作用：燃燒過程會產生一氧化碳同埋煙灰。

溫度掛=,=
我記得唔同色的火有唔同的溫度,
等D專業人士答先