Bob, you are wery right about eaven trace amounts of minerals on changeing flame colour dramaticly. Some more thain others. Such a example is sodium.
We had a flame spectrometer at school, basicly it injects a liquid sample in a propane flame and reads out the colour change and enables you to determin its composition and concentration. Once someone spilled some sodium containing solution on the machine and it never worked right again. Trace amounts of sodium stayed on the burner. Same can be seen if you spill soup on a propane stove burner. Intense yelow orange flame indicates sodium.
This sayd, tar can be a sneaky thing too. Sometimes trace amounts can be seen in the flame, sometimes it all burns blue while glueing the pipes such was the case with me the other day. I took apart some pipes from my Mercedes gas suply (after the sack filter) and every seam was glued with droplets of solidified tar. Those pipes never eaven had woodgas in, just chargas! It is possible a couple of brands sneaked in the fuel. This is why l decided to go downdraft now.