That is a genuinely difficult question to answer. I would guess that the color is the result of residual minerals displaying at a different ratio, depending on the combustion temperature. But that is just a guess.
The quote below is with regard to wood ash, which may be a close proxy:
"The study has found that at raised temperature K, S, B, sodium (Na) and copper (Cu) decreased, whereas Mg, P, Mn, Al, Fe, and Si did not change relative to calcium (Ca). All of these trace elements are, however, present in the form of oxides at higher temperature of combustion."
en.wikipedia.org
Bob