A diesel engine will produce three types/colors of smoke out its exhaust; black, blue, and white. Black is usually the most common. Causes of black diesel exhaust may be; carbon deposits in combustion chambers, dirty or worn fuel injectors, incorrect fuel injection timing, poor fuel, sticking piston rings, turbocharger pressure too low, and many other causes too numerous to list here. Blue diesel smoke may indicate cylinder glaze, piston ring wear, incorrect grade of oil (too thin), and/or worn valve guides and seals. White smoke coming out the exhaust of a diesel truck is generally a little easier to diagnose. If the white smoke only appears at start-up, then disappears as the engine warms, then there is either cylinder glazing or deposits around the piston rings. Low cylinder compression and bad fuel injectors can also lead to the white smoke. You should also check fuel injection timing. The worst case scenario is water getting into the combustion chambers, which means blown head gasket or cracked cylinder head/blocks. Hopefully, it is one of the more simple reasons stated above!