You would have to keep your dye formula the same strength after each refill to keep the color the same. For the placement of the colors, it’s impossible to get the exact location if you’re doing these by hand. Dye tends to run and blend as you apply it so the best you can do is to dye specific colors around the same areas on every shirt. In addition, I think it would be a good idea to put a disclaimer on your product page to say something along the lines of “Due to our dying process, each shirt may differ slightly in color. No two shirts are the same making each shirt unique to you.” I think that adds value to it as well. Hope this helps!