The flame produced by liquid fuel is more luminous than a gas flame. For a given heat release, a liquid fuel flame will be longer and of a less predictable shape than a gas flame.
Both blue and yellow flames produce the same products of complete combustion: carbon dioxide and water, so there is no difference in the amount of heat released.
Yellow flames result when fuel dissociates (cracks) to its carbon and hydrogen components. In gas burning, this is more likely to happen on the heavier gases like propane and butane, particularly when poorly mixed with air. The higher the hydrogen/carbon ratio by weight in the fuel, the less prone it is to yellow flame burning. Also, the lower the molecular weight in the same series of compounds, the less prone they are to yellow flame burning. The separate burning of the hydrogen is very fast and that of carbon slow. The slower burning free carbon is heated to a bright yellow incandescence.
Oil flames are typically characterized by a very bright yellow incandescence because there is so much free carbon around. Somewhat yellow flames are typical of the heavier gaseous fuels, too, but yellow flames in gas burning are more often a tipoff of poor or inadequate air mixing.