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Originally Posted by winny

Flame colour has nothing to do with the angle you view it from or the length of the exhaust pipe, it is nothing to do with temperature either. It simply shows how efficiently the fuel is burning.

If the combustion is burning all the carbon you get blue flames and no smoke. If there's carbon coming out in the exhaust gases then you'll get yellow-er/smoky-er flame.

This is chemistry not opinion.

Originally Posted by changai

Actually, Winny is right, it doesn't depend on those factors.
Color is related to the temperature of gases produced during combustion: the hotter, the whiter; the colder, the redder. Blue indicates a very high temperature. Near-perfect combustion of hydrocarbons is always blue.

Yellow indicates an imperfect combustion, i.e. lack of combustive agent (usually air) which causes production of soot, i.e. smoke. However, even a very rich mixture as used on a cold engine would not produce yellow flames, but add a yellowish hue at the end of blue flames.

Red indicates a very bad combustion. A damaged engine burning oil would probably produce reddish flames.

Hope this helps

these people seem to make sense