White vapor from the breather tube (particularly on the CB500/550/400F/350F) usually indicates incomplete combustion - especially if there's ethanol in the gasoline. The hydrocarbons nab the nearest water molecule when they get hot but aren't burning, and a white fog appears for a few seconds from that exhaust pulse. This is why a car usually blows white (much more briefly with EFI systems and modern computer-closed-loop combustion systems) when starting on cold mornings.
If the engine is running rich, then this will continue longer: the crankcase vapors linger the longest, especially until the rings seat fully to drop that to a minimum. In the 550 Honda finally had to route the engine's breather back into the airbox (lower left corner) to satisfy the EPA circa 1975, as the EPA then claimed it was "...dumping unburned fuel into the air [sic]..." circa 1974 testing. Ugly stuff...
Most of the time when I see this on startup following a top-end rebuild, but without a complete bottom-end cleaning (like in a machine shop parts washer) it is due to old oil (it absorbs water from the air, when in use) harboring corners of the crankcase: thus it goes away after an oil change or two with consistent riding. If the bike is [again] stored, like over winter, in the Spring it puffs white awhile during the first few runs: my 750 does this every Spring because I change oil in the Fall when parking it, so the oil has time to gather some water over the winter - which is normally our 'wettest' time here in Colorado (except for this year's drought conditions, I might add...).
When I see it in an engine startup that I fully rebuilt AND cleaned completely (usually the 750), it can indicate the carbs need work: they are running too rich. This is very common with the 750's roundtop carbs, which run rich below 2500 RPM anyway, unless rejetted to 37.5 idle jets and the float bowl lowered a bit. In the CB500/550 before the PD carbs came out, this is the same story: the low end RPM (up to 3300-ish) is quite rich, so it promotes the condensate smoking. This doesn't seem to happen as much in the later CB550K3, but that one has uniquely stingy carbs (PD46 series) and associated cold-run behavior unless they are retuned a bit.
