When the air intake openings are completely closed what color is the flame?

When the air intake openings are completely closed, the color of the flame is expected to be yellow. This is an indication of incomplete combustion in the burner. For combustion to happen there should be enough oxygen that is supplied to the system. An indication of a complete combustion would be a flame that is blue. As the openings are closed, there would be limited supply of oxygen so the combustion would not only produce CO2 but also CO which is the cause of the yellow color of the flame. CO particles or soot particles cause incandescence to the flame causing it to appear yellow.