Which granites are harder, black, red or salt/peppers?

There are many types of granites. The black granites are generally softer than others. The granites that wear easily are much softer then those that appear to last forever.

FYI There are many types of granites and some fall into a category geological known as gneiss granites. These granites go through a metamorphic process very similar to marble. If this peaks your interest, you may want to consult a geology text book for further information.