White smoke with a cold engine means fuel is not being burnt and goes out the exhaust, assuming the engine is healthy. A cylinder that is mis-firing will give steady white smoke, or puffs of white as it begins to fire properly. When compression is getting lower, this is more prevalent at start-up. Of course ambient temps have a big part in this. It will stop smoking white when the engine fires properly on all cylinders.
Black smoke is indeed too much fuel for the air, for whatever reason - too much throttle for the RPM (as when the tractor is taking a load and the RPM's drop but the throttle stays the same), dirty air filter, plugged exhaust, build-up on the valves (not really an issue), faulty injectors or pump, etc.
Blue smoke is oil burning, same as a gas engine. This is usually noticed with oil consumption. Worn pistons/rings/cylinders would be the most likely culprit. This is accelerated by infrequent oil changes, or faulty air filter or intake hose from air filter. Father-in-law, a 40-years-experienced tractor mechanic, told me he once did a complete motor job for a farmer with a Massey 245. The farmer installed the engine himself (??? to save $$ I can only guess), didn't hook the hose up properly to the air filter, was doing some plowing in a very dusty field, started in the morning, stopped for lunch, and the tractor wouldn't start again after lunch because the cylinders were worn out. Dust makes a fine sandpaper. Not bad....5 hours on a fresh motor before it had to come apart again. Extreme case, but it shows what dirt can do to cylinders.
White smoke when warm is water in fuel (often noticed with a misfire) or coolant, which is steam. This usually doesn't carry far in the air like smoke does. Dissipates quickly in the air, unlike smoke that will carry a distance. Coolant can come from head gasket, cracked head or liner. Teardown required.
Hope this helps somewhat.
