It is empirically established that the flame color is closely related to the combustion phenomena occurring in the flame. For example, the color of a nonluminous premixed flame changes from blue to blue-green as the equivalence ratio increases. This phenomenon is caused by the change of the chemiluminescence property of active molecules formed in the elementary reactions. On the other hand, the color of a luminous diffusion flame changes from yellow-red to yellow, depending on the temperature in the flame. This is due to the change of the thermal radiation property of soot.