In general, I'd avoid black, for a number of reasons,...heat in the summer, dust/dirt, fingerprints, and the general difficulty keeping it clean. Black, under low-lighting conditions, also makes it more difficult for other drivers to see it at night...at some angles where the headlights and taillights don't show.
You seem concerned about the vehicle getting dirty fast. If that's the case, I'd probably recommend the Satin Cashmere Metallic. It's a color that seems to blend in almost perfectly with dust, dirt, scratches, and the general stuff that accumulates on a car's finish day by day. It will hide a lot of the daily dirt-build-up, and you won't have to wash it as often. It's not an exact match for tree-pollen (which tends to be yellowish-green), but will hide it better than some of the other colors. Of course, silver or white will probably be best at hiding white or light-gray salt-stains in the winter....but you don't want salt to stay on the car very long, anyway. And white, though nice and cool in the summer, makes it harder to heat up and melt ice/snow on it in the winter.....it also tends to show colored-stains quite easily.
http://www.lexus.com/build-your-lex...d/112/my-dream-car/{-x-:-04U7-,-i-:-EA24-,-a-:[],-p-:[],-z-:[]}
In the end, of course, it comes down to which of the available colts is best for your needs, driving style, and cleaning habits.
And, whatever color you choose, get a bottle of this: It's inexpensive (generally $4-5), effective, and will remove all kinds of surface (not deep) paint-problems, keeping the paint just like new. I use it myself. (make sure it's the SCRATCH-OUT scratch-remover and not the rubbing compound).