buying live rock is the most important part of the reef process as it will be your reefscape and your biological filter.
the region the rock comes from will play into its shape and density - some areas have very dense rock and others have very porous. some regions like tonga are famous for their branch rock and figi has awesome shapes and caves - choose what you like the look of
when buying live rock you really don't need to ask anything but do this:
1) look for pests like aptatia and valonia
2) choose by the shape and size more then color - coraline will grow eventially
3) inspect their holding bin - does it have good flow, does it look like they are curing it properly - uncured rock will smell bad, cured rock smells like ocean. although there is nothing wrong with uncured rock and many prefer to buy it that way and can cook it yourself
4) i prefer to buy larger pieces rather then a bunch of small stuff and then try to stack it, an average nano - 1 to 4 pieces
5) buying from internet places like LA only makes sense if you are buying a quantity of rock otherwise it is far nicer to hand pick your own even if it means paying a bit more
6) then even if they tell you their rock is "cured" - cook it with the two bucket method anyway....... so many fail to do this very important step and that is why they end up with aptatia, crabs, valonia, flatworms, and other nasty hitchhikers