It depends on the temperature of the reaction, and its byproducts.

The classic orange flame is produced by tiny nonreactive particles (soot) heated to around 2,000°F. They glow for the same reason all hot things glow: they give off blackbody radiation. Blackbody radiation gives off radiation at a wide range of frequencies, but peaks at a range determined by the temperature.
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Some things burn at low temperature and some at high temperature. so it varies in temperatures but will burn at a accurate temperature and it also depends on the material used that it gives out the fuel to burn . so dried leaves burn while iron doesn't