Granite is a universally hard and impervious stone that makes beautiful counters. The color comes from mineral inclusions and varies from jet black to nearly white. The color does not affect permeability, but there is some variation in the crystalline structures that may. To guard against staining and enhance the finish it is sealed. I use SCI (Stone Care International) products including the solvent based Stoneguard Sealer (about $49 per quart). You will need less than 1-cup of sealer to finish the job because the granite is so impermeable. I use the product on marble and travertine as well.
The company that sells or prepares your slab will probably have their own sealer recommendations. In fact, why not ask them. It is possible the stone will be sealed as part of the installation service.
Other than that, go with whatever color makes you happy, because you won't be changing the stone anytime soon.