I have been told that darker marbles are softer than lighter colored marbles is this true?

No, it is absolutely false. There are soft, dark marbles and there are hard dark marbles. For example, negro maquina is a very soft marble. On the other hand Vermont Green is a very hard dark marble. Do not rely on color alone to determine the hardness of marble. It is best to perform a hardness test to verify hardness.

TIP A simple hardness test is to use a small pocket knife. Run the knife lightly across the surface of the stone. If the stone scratches its is relatively soft. If it does not scratch, it is hard.