White exhaust smoke is usually the result of condensation and moisture buildup in the exhaust system; it can also be the result of burning coolant within the engine. When white exhaust smoke is thin and vaporous, it usually does not signify a problem within the car.
If the exhaust smoke is thicker and comes out in large clouds, it generally means that there is something burning within the car. A blown head gasket could be a possible reason for coolant burning in the car; a cracked engine block is also a reason for coolant burning in the vehicle.
Gray smoke that is coming out of a tail pipe is usually a major cause for concern. Gray smoke often signals that there is a problem with the oil system of the car. The car may be burning more oil than necessary and may be running hotter than it should.
Black smoke coming out of the back of a tail pipe signals that too much fuel is being used and burned up. If the vehicle is burning too much fuel, it should be fixed to ensure that money is not being wasted on fuel that is simply burning up and not being used to power the vehicle. Diesel vehicles are the most common vehicles to have black smoke come from the exhaust pipe.