Any colour or clear. Go to the cheap end of line bin at Priceline or chemists warehouse or similar and get a bottle. Probably won't cost more than a couple of bux. There might even be a colour there that's close or use a colourless one (top coat). There's a tiny brush inside the cap for applying it. Easily removed with acetone when the time comes to fix the car. Just tell the mechanic or spray painter or whoever that you used nail polish on the bare metal if any. One coat should be sufficient to protect against rust, but do maybe two if it'll be a while before getting the damage fixed properly.