The different chemical elements burned/mixed in with the powder.

I think, there are different metals in the powder and they burn in different colours

Different metals in with the explosives (gunpowder, whatever...) provide the color when they burn. Copper is common, used for green. Like, butane lighters that turn green do so because of a little strip of copper that the flame burns/oxidizes. A quick google search of "firework color" should help you if you want to find out which other metals make other colors.

natural dye/color in dry powder form...from a cake decorating section at Michael's.

Firework Color Production & List of Colorants Creating firework colors is a complex endeavor, requiring considerable art and application of physical science. Excluding propellants or special effects, the points of light ejected from fireworks, termed 'stars', generally require an oxygen-producer, fuel, binder (to keep everything where it needs to be), and color producer. There are two main mechanisms of color production in fireworks, incandescence and luminescence. Incandescence Incandescence is light produced from heat. Heat causes a substance to become hot and glow, initially emitting infrared, then red, orange, yellow, and white light as it becomes increasingly hotter. When the temperature of a firework is controlled, the glow of components, such as charcoal, can be manipulated to be the desired color (temperature) at the proper time. and date the firework was produced greatly affect the final display (or lack thereof). Firework Colorants Color Compound Red strontium salts, lithium salts+ lithium carbonate, Li2CO3 = red strontium carbonate, SrCO3 = bright red Orange calcium salts calcium chloride, CaCl2 calcium sulfate, CaSO4·xH2O, where x = 0,2,3,5 Gold incandescence of iron (with carbon), charcoal, or lampblack Yellow sodium compounds sodium nitrate, NaNO3 cryolite, Na3AlF6 Electric White white-hot metal, such as magnesium or aluminum barium oxide, BaO Green barium compounds + chlorine producer barium chloride, BaCl+ = bright green Blue copper compounds + chlorine producer copper acetoarsenite (Paris Green), Cu3As2O3Cu(C2H3O2)2 = blue copper (I) chloride, CuCl = turquoise blue Purple mixture of strontium (red) and copper (blue) compounds Silver burning aluminum, titanium, or magnesium powder or flakes Hopefully that gives you your answer! (I got this info at About.com)

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just get pissed