Lighter granite is more porous than darker, and black is one the most dense stones out there. Every granite is its own individual stone with unique characteristics, density, viscosity, etc. Apple Pie Order gives some good advice, but I would seal the samples first (except the quartz) or at least part of the sample so you can see the difference. As long as you seal the stone you shouldn't have any issues. A lighter granite you will probably just have to seal more often (maybe once a year). You'll know it's time to reseal when the stone starts absorbing moisture from, say, condensation from a cup of water. If the stone actually darkens and the water just doesn't bead up, it's time to reseal. You can also look at the area around your sink, if it starts darkening with the water, same thing. I don't see any issues with using a lighter stone on your island.