The color of a luminous flame is yellow. This is because of small soot particles in the flame that are heated to incandescence. Producing a luminous flame requires either a shortage of combustion air, as in a Bunsen burner, or a local excess of fuel, as with a kerosene torch.
The flame is yellow because of its temperature. To produce enough soot to be luminous, the flame is operated at a lower temperature than its efficient heating flame. The color of simple incandescence is due to black-body radiation: as the temperature decreases, the peak of the black-body radiation curve moves to longer wavelengths. Other factors, particularly the fuel chemistry and its prevalence for forming soot, have an influence on luminosity.
The Victorian chemist Edward Frankland studied luminosity in flames and discovered that their luminosity also increased with pressure.