Are you using pre-mixed fuel from a reliable source, or are you mixing your own fuel? There are different grades of caster and various synthetic oils, and different oil mixes will change the exhaust residue color a bit.

However, black (not dark brown, black) comes from metal wear or from really burning the oil up, both of which are bad things.

Very dark brown is also often a sign that you are burning oil up, meaning that your engine is running too hot (usually, too lean) and burning oil. Or, if you mix your own fuels, or are using a no-name fuel, it could, in fact, mean poor quality oil as your friend suggested.