The heat and light in the flame are generated in the reaction zone. The flame appears yellowish if there a large numbers of soot particles, which get hot and radiate heat. The flame appears blue if there is little or no soot. Flames in camping stoves and gas cookers are blue because a well designed burner produces a premixed flame with very little soot. Flames in camp fires and in candles are yellow, because the oxygen and fuel must diffuse together in the flame.
All fires start with an ignition.
If there is insufficient energy, the molecules do not react with each other. The energy to cause ignition might be in the form of the heat from a match or a spark or a cigarette. The energy makes the molecules of fuel and of oxygen nearby move faster. This energy is called kinetic energy. If they move fast enough, they will react when they collide.