Since their inception in China, black powder has been the main ingredient in fireworks. Since the original intent of fireworks was to scare away evil spirits, the loud cracking sound was more important than the color in the beginning. Black powder will, however, give off a bright yellow color. The chemical makeup of black powder is 15 percent charcoal, 10 percent sulfur and 75 percent potassium nitrate.
To create colors, fireworks makers rely on their understanding of how certain combustible chemical compounds react during the energy transfer that comes with heat. For instance, copper acetoarsenite and copper chloride both turn blue; lithium carbonate and anhydrous strontium carbonate both turn red; barium carbonate and barium chloride turn green; and cryolite turns yellow. Various magnesium and aluminum compounds create bright white colors.
When energy is released from the chemical compounds previously discussed, they create wavelengths that we see as color. Compounds releasing higher energy correspond with shorter wavelengths and colors on the blue side of the color spectrum. Compounds that release lesser energy correspond with longer wavelengths at the red side of the color spectrum. If you have hear about ultra violet or infrared, these are colors that are too short and too high, respectively, to be seen with the naked eye. Firework colors, by necessity, fall between the extremes.
Knowing which compounds create colors isn't enough to create fireworks. For fireworks to function properly, they need to be created in a way that dictates the patterns they make and gets them hot enough to burn and display colors. To do this they are arranged in to 3 or 4 cm cubes called stars. These stars contain an oxidizing agent, a reducing agent, the coloring compound and binder that hold it all together. Oxidizing and reducing agents vary based on the type of action desired from the firework.