One of the by products of combustion is water. Also, any humidity in the surrounding atmosphere will condense on surfaces that have a temperature differential from side to side. Your cold drink glass is a common example. When the muffler cools down the water condenses inside the pipe. The heat of the next run cycle vaporizes this water and it exits as white vapor. Also, choke operation causes the engine to run so rich that the combustion is not complete. The incomplete burn residue exits the muffler as black soot suspended in a gaseous medium. Mixing this with a white vapor cloud will give you grey smoke.
Something to note is that the muffler heats from the exhaust valve to the pipe exit. If the run cycles are short and the flow is small (idling), the entire muffler may not reach a temperature high enough to convert all the water into vapor and dry out the muffler internals. Repeated short run cycles can build up more water in the muffler than can be exhausted. This is a major contributor of exhaust systems rusting out from the inside.
Lastly, the severity of the condensation is proportional to the magnitude of the temperature extremes. Or, you'll see more vapor in cold weather than in hot weather.
The only thing to be alarmed at is if the exhaust vapor has a bluish tint, as this would indicate oil in the combustion chambers. If this only happens during start up, then valve guides/ or guide seals are suspect.
Cheers,
