Paint chemistry is certainly not my area of expertise. I would suggest working with your local paint company and they should be able to assist you. I write that because I always just assumed to make something a darker gray you might just add black - but my experience with my paint company has taught me that it depends on the paint base, other colors in it, and other items that I just don't understand. My paint company has always been able to help me get just the right colors - and they are the real experts in that area. Good luck. John