Some firework colors are safer than others. You can get silver and gold from heating charcoal or metals, such as aluminum or titanium, where the glow comes from incandescence. Other colors are produced by burning metal salts. Colors work a lot like the flame test, where the color actually produced results from the interaction between the emission and absorption of parts of the spectrum by the various components of the mixture. In general, metal salts burn in characteristic colors, but there are relatively non-toxic metals that can be used to produce any color of the rainbow. If your fireworks come from a reputable manufacturer, losing one in your yard won't produce Teenage Mutant Ninja clover. Keep in mind, there may be less expensive alternatives in use by other manufacturers that aren't as safe.