I'm working on the same project also.
I found a place called Parasol Inc. that sells dye color for rubber, plastic, and vinyl items. There listed on the net as (
www.parasolinc.com). They have a few different brands of dye depending on what type of material you have. I'm sending them a sample piece of reveal molding so they can determine which dye brand and color shade of gray I need to use. This stuff cost $96 a quart, that's why I'm going to let them make that determination. Should have more info on this in about a week.
Mike G.