With different smell and color, the smoke sometimes has visible density. The reasons behind this phenomenon are several, but principally there are distinctions relative to the color.
Smoke with a black color possibly indicates too much fuel passed to the combustion chamber. If the smoke is partially blue, it means that the vehicle is passing oil to the cylinder to explode with gasoline. That's not the idea in four cycle engines, but two cycle ones, so it indicates a fault in the normal behavior of the motor.
Gray smoke can lead newly to guessing an incorrect burning of oil at the cylinder or maybe some malfunction in other pieces like catalysts, valves, turbos, … Other possibility is when the smoke is white, normally meaning simply mixed condensation.
Any of these imply increasing the degree of air pollution, and for those who don't care specially, interrelated fuel and oil costs. Thus, they should be reasons to check it with specialists for solving the problem.