Hello Blesmee,
It is mainly due to different ratios of oxygen to the combustable gas. An excellent source of information on the subject can be found at . The following picture and explanation is taken from this article:
Different flame types of a depend on oxygen supply. On the left a rich fuel with no premixed oxygen produces a yellow sooty diffusion flame; on the right a lean fully oxygen premixed flame produces no soot and the flame color is produced by molecular radicals, especially CH and C2 .
LeeH
(published on 07/06/2014)
