While a luminous flame burns yellow, a non-luminous flame burns blue; this is because of the difference in available levels of oxygen. Luminous flames have limited access to oxygen, while non-luminous flames have unlimited access to oxygen.
Flames work by combining their own carbon molecules with oxygen molecules in the air to form carbon dioxide. Luminous flames do not get enough oxygen to turn all the carbon that is being burnt into carbon dioxide. Some of this excess carbon is released as soot; this is why luminous flames produce soot, while non-luminous flames do not. Because non-luminous flames are able to combine all their carbon with oxygen, they burn far more efficiently and are much hotter than luminous flames.