This can seem a simple question with perplexing answers<br>
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White smoke is un-burnt fuel oil being emitted out the exhaust; due to the size of the un-burnt fuel droplets they give the appearance of white smoke, not to be confused with blue smoke<br>
In some cases blue smoke may appear whitish (light/white back ground) an easy way to establish which is whish is to smell the smoke Blue has and burnt lube oil smell, were as white smoke has a strong sharp odour that may sting the eyes.<br>
Now back to the causes - If you crudely equate white smoke similar to steam, white smoke is the product of "cold combustion" this can be caused by many reasons - lack of compression, retarded injection timing etc<br>
In the "possible reasons mention above it must be mentioned that each symptom may have various contributing factors - such as lack of compression, may be due to worn piston compression rings (may also have hard start problem), equally this may be caused by worn tappets (poor engine breathing) or even a faulty coolant thermostat<br>
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Black smoke is equally allusive, again the product of un-burnt fuel emitted but possible due to such factors as advanced injection timing, blocked air cleaner, worn camshaft, excessive fuel quantity, injection pump requiring service, or even injectors requiring service.<br>
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Again these symptoms have to assessed with the engine condition, such as operating temperature, engine speed, load atmospheric conditions, general engine condition and the nature/colour of the smoke. <br>
