A yellow flame is achieved by allowing the gas to use some of the ambient air for combustion. A lack of a yellow flame is caused by having too much air in the air/gas mixture.
To fix this problem reduce the amount of air mixing with the gas prior to the burner. Typically, this is caused in liquid propane installations where the air mixture valve is oversized for the burner size. You can test this by temporarily blocking some of air intake into the valve, if this solves the problem replace the valve with one with a lower BTU rating.
Air shutter setting incorrect – check to see if too much air is being mixed with the gas.
Crack in burner gasket – check for leaks allowing air into the gas