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From PopSci.com:
Wood fires are yellow, right? Well, typically, but not necessarily. All hot bodies emit "black body" radiation in a color spectrum that depends on temperature: red for fairly hot, orange for hotter, yellow for really quite hot, all the way up through white for, well, white-hot. Most wood fires burn at about yellow-hot.
But there's also a second reason for the usual yellow color: something called the "spectral emission line." Wood (along with most natural substances) contains sodium, which produces a yellow emission when ignited, regardless of the temperature.
If you mix substances with different spectral-emissions lines, they'll burn different colors. Pyrotechnicians have a long list of chemicals that emit particular colors--combine lines with enough gunpowder to make a big bang and . . . boom! Fireworks are born.
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