|Quoting Ceph (Reply 3):|
Blue or non-luminious flame are produced when there is complete combustion. Orange or luminious flames are produced when there are incomplete combustion as there are still carbon around.
Unrelated to aviation, but this information has saved lives.
Some years ago, a young girl had learned the above fact at school. At home that evening, she looked through the little window of her parent's gas heating boiler and noticed that it was burning yellow. Thinking it odd, she pointed it out to her dad who in turn called a repairman.
Turned out that it was pumping out Carbon Monoxide by the bucket load because of blocked filters reducing airflow.
So, yes, it probably is nothing more than varying degrees of combustion.