It all depends on what you are adding the black for. If you're trying to make a light blue to a darker blue or a green a little darker. Maybe you just add 1 drop of black to 4 oz. plastic and a little black glitter to make a very light smoke.
You can change white to grey(not silver) or regular orange to a burnt orange and then add a little red highlite for a really nice effect.
You've heard the phrase "basic black"? Well black is a basic color and will enhance or slightly alter any color. The amounts you use depends on what you want to come out with.
If you can afford it; just play "mad doctor" and start with each color you have then add 1 drop of black to see what happens. (keep notes on what you do, when you do it, so that you can repeat it) Sometimes you will amaze yourself at what comes out when you play around with colors. You will, unfortunatley, ruin some of the batches you make; but this becomes a learning curve.
In any case, take a day and try it.