You could make a value scale on sprayout cards. Get some small mixing cups, mix some epoxy 4 parts white to 1 part black and spray a card. then mix 3 parts white to 1 part black, spray, 2 to1, spray, 1 to 1, spray, 2 parts black to 1 part white, spray, all the way to 4 black, 1 white. Use these cards as a reference to the value of the color you are spraying. For instance, say you are spraying a gunmetal, mix your paint with a stick and hold it up to the cards that are close. Choose the epoxy color that matches the color the closest. Yellows and oranges like white, or very light gray. Reds like light to med. light gray. Bright blues like med. gray. Darker blues and greens like a med. dark gray. Dark blues and charcoals like a darker gray. Using sealer and/or primer like this will save a lot of extra coats of base (therefore money).