White is usually water and / or coolant. Water out of the exhaust is completely normal and shouldn't be a cause for concern. Since it only does it on startup and doesn't last more than a few minutes, I wouldn't worry too much about the white vapor as it's most likely just condensation / water.
If it becomes continuous, then you might want to start worrying about it since that's generally a sign of a leaking / blown head gasket or other internal engine problem that allows coolant into the cylinders.
Quick science lesson if you're interested in why water is normal:
Gasoline is made up of carbon chains of different lengths ranging from C7H16 through C11H24. Although gasoline contains many different chemical compounds, it is made up mostly of hydrocarbons, and all hydrocarbons form the same products when they are burned (just in different amounts). When a hydrocarbon is burned (that is, reacted with oxygen), it forms carbon dioxide (CO2) and water (H2O).
For our "average" gasoline of C8H18, the reaction is 2 molecules of octane reacting with 25 molecules of oxygen (O2) to form 18 molecules of water and 16 molecules of carbon dioxide. Of course, this reaction only occurs completely in an ideal world. In the real world, there is usually not quite enough oxygen available fast enough inside your car's engine to allow the reaction to occur completely, so there is also some carbon monoxide (CO) formed as well. In addition, since the oxygen is provided by bringing air into the engine, and since air consists mostly of nitrogen, some oxides of nitrogen (NOX) are formed as well. Finally, some of the trace elements in the gasoline (such as sulfur) can react to form small amounts of other pollutants, such as SO2.
So, to sum up, gasoline is a complicated mixture of hydrocarbons boiling between 120 and 400 degrees F, with chemical formulas between C6H14 and C12H26, but a good "average" compound is C8H18. These react in an ideal situation to produce carbon dioxide and water, but in an actual automobile engine they also produce some amount of undesirable compounds including carbon monoxide, oxides of nitrogen, and sulfur-containing compounds.
