Limestone can come in a variety of colors and hardnesses. There should not be any reason why you can't use the darker colored. It is, of course, harder than the white or yellowish stone. There were many times that I slid down grey limestone slopes as a geology student in New Mexico. I believe I left a little of myself behind. Test it first with a knife blade ( scratch a little powder up) and place a drop of vinegar on it. If it fizzes, it's limestone.