The flame (propane) on our 10 yr old Valiant boiler I(Has 3 zones) is sometimes burning yellow not blue. There are two burners. Each burner has a long grated area inside through which the fuel is burned. The flame looks correct most of the time but occasionally
The blue flame occurs in the tube (looks like a rocket thruster) that is outside the interior of boiler)i and the grated area has a yellow flame Still have heat. I have not noticed this in the past just recently. Someone said the air needs to be adjusted. Also there is a different sound to the burning process when this is happening

A number of things, gas (firing) pressure needs to be checked to factory spec, and burners need to be cleaned... and lastly, a good contractor can adjust them to burn MOSTLY with a blue flame... better yet checked with a combustion analyzer to see how the unit is really burning such as CO (carbon monoxide etc.)