You would need to mix in some black paint (or more simply some black colorant). The amount would depend on just how dark you think "slate gray" is. You would probably need to do some experimenting. I would also recommend mixing all the paint together to get consistent color. Since that would be one big vat of paint, it might be easier for you to go to a paint store and ask them to make a match for you. They could add colorant to each gallon in a consistent manner and convert that paint from white to gray for you.