1. Colourless or slightly blue smoke from exhaust
This means that your bike is very good condition. What you can assume is your fuel/air mixture is at the right composition and everything is well burnt is your combustion chamber.

2. Yellow or brownish smoke from exhaust
Your fuel/air mixture is too lean. Too lean means that there is low fuel but high air mixture in your combustion chamber.
Solution: Adjust the fuel/air mixture at your carburetor (or ECU) accordingly.

3. Black smoke from exhaust
Your fuel/air mixture is too rich. High fuel but low air intake. This will reduce your fuel efficiency since the black smoke is your unburnt fuel.
The problem might be at this part:
i. Your air filter is dirty or blocked from receiving air from surrounding.
ii.Your choke is closed
iii. Your carburetor is damaged
Solution:
i.Clean/replace air filter
ii.Open choke
iii.Repair/replace your carburetor.

4. White smoke from exhaust
This happen when your engine oil gets into the combustion chamber.
The problem might be at this part:
i. Piston ring is loose
ii. Piston is worn out
iii. Engine block is worn out/scratched
iv. Valve is worn out/damaged
v. Valve line is worn out/scratched
Solution:
i. Replace piston ring
ii. Rebore block and replace piston
iii. same with no. ii
iv. Replace valve
v. Rebore valve line and replace valve.
Sometimes white smoke can also occur in the morning or you start your bike after it is being left on the rain. The white smoke is the water vapour released from the condensation that happens inside your exhaust. Normally, this happens only for few minutes and then it will be back to colourless again. The difference from white water vapour smoke and white exhaust smoke is the smell. What I can say is, the smell from engine oil being burnt is your combustion chamber is bad and very busuk. Water vapour doesn’t have any smell.
For more information, you can search in Google.