The most common reason why a pilot light would start to burn yellow is that the air intake valve to the pilot light is clogged with dirt. When this happens, air will be blocked to the flame and it will suffer from incomplete combustion (causing the yellow color). If there is a small amount of dirt blocking the air, the yellow color might only be at the tip of the flame. If there is a large amount of dirt, the entire flame will burn yellow.
Another issue with a yellow-burning pilot light is that it allows carbon monoxide to leak into your business’s air. That’s because carbon monoxide is not completely burned up by the lazy flame, and the excess gas will simply escape.
The issue can typically be fixed by cleaning out the air intake valve to the pilot light, which should only be done by a professional who knows how to access and repair the part.