This is quite a debatable subject and is open to many different different interpretations.
There are noticable differences between direct and indirect injection diesel engines with the direct injection engines normally being the cleaner burning of the two genres.
During normal combustion diesel engines form soot and carbon particles which gets exhausted from the exhaust during normal operation.
Smoking becomes more visible at night purely due to the amount and spectrum of visible light available (vehicle headlights, street lights etc). Some haze is perfectly normal and does not neccesseraly indicate a problem.
A well tuned diesel engine in good condition will show limited amounts of smoking during normal operation, throughout the rev range when not under load (such as when being revved up in neutral). Once "on boost" when placed under some form of load the smoking should be minimal, preferably not noticeable at all. When placed under high torque demands some grey smoke may become visible.
White smoke can indicate unburnt fuel and / or combusted oil. Some smoking (white or grey) is acceptable during start-up and some period thereafter until the engine's combustion chambers have heated up sufficiently.
Grey smoke is normally an indication of incomplete combustion and some measure thereof is totally acceptable during hard acceleration.
Black smoke is a sign of overfuelling and is undesireable.
White / grey smoke during acceleration after a period of decelleration can indicate, as with a petrol engine, worn valve stem seals. White / grey smoke during hard acceleration can indicate worn rings.
Excessive smoking can be caused by dirty air cleaners (grey to black), worn injectors (white, grey or black), low boost (black), poor engine condition (white, grey or black) or incorrect engine timing.
Rule of thumb - None, or at the most, a very light amount of sooting on the rear of the vehicle after a long trip or some hard driving is acceptable. Heavier deposits may serve as indication of one of the elements mentioned above being out of kilter and might need some attention.