They will have to be one color unless you get them done your self. Like I said earlier cost goes up significantly, these are die cut out of a roll of vinyl so if you get multiple colors each part that is a different color has to be hand placed.
As far as black not showing up on a black sled, the only reason I made them black and white is because the black parts represent vinyl, the actual color of the vinyl can very. It will be cheapest if we stick to simple colors and get a bunch ran of each, depending on how many people want, 10 red, 10 white, 10 black, 10 blue for instance.
If you want anything different you will have to have it made your self, I can design it for you and give you a working file but you will have to have it cut, it is cheaper to do batches of the same sticker instead of 50 individual stickers.
I am betting the cost will be around $7 each but it will depend on how many we want. They will have to be purchased all at one time, I am willing to front the cash but will need assurance that they will be bought. Obviously I am not going to take a cut.
I may be able to work out a deal with them where they can just hold the design and if you want one you can call them up, have it cut and pick it up/pay for it your self but that might not work, I'll talk to them after we figure out how many need to be made.
The font I used was Impact.