John; When I want to make a color that is not a stock on the shelf item, I use acrylic paint in small bottles from the local Hobby Shop. I use a sheet of wax paper to blend my colors together until the color I am looking for is found. Use a note book to keep track of the amounts of drops used to obtain the color so if you ever want to repeat this somewhere down the line you can. Skiprat is correct, Don't add white to black in order to achieve gray. I would start out with about 10 drops of white and add 1 drop of black. Mix well and see what you have. Now you know if you want a lighter shade of gray (add one drop of white at a time), or a darker shade (add 1 drop of black). You could also pick out a shade of gray you like and just add that to your casting. Powders may be hard to repeat if not measured properly. Same with dyes you are mixing. Paint is cheap when you are experementing with color mixing. Jim S
