The essence of flame is a high-temperature gaseous or plasma substance. There are two factors that determine the color of the flame:
1. The temperature of the flame determines the color of the flame. Flame is a reaction (violent flame). When the temperature is low, it is infrared. With the increase of temperature, the flame ranges from red-orange (3,000 degrees) to yellow-white (4,000 degrees) to cyan (5,000-6,000 degrees) to purple (above 7,000 degrees) and finally invisible ultraviolet (tens of thousands).
From the point of view of high-energy physics, the flames in infrared and color spectra are low-energy flames. With the rising temperature, the color of flame, from ultraviolet rays to X rays to gamma rays, is an indescribable "color".
2. The elemental composition of gaseous and plasma substances determines the natural spectrum of flame. Each element in the element list will emit its own specific light color at high temperature. For example, sodium appears yellow, potassium is purple, copper is green, and the light color of the compound is variegated, because there are many kinds of luminous elements.