Okay, foolproof method for beginners number 1;
Paint the wheelhub in whatever colour the instructions say, try not to get too much paint on the tyre.
Push a wooden cocktail stick through the hole in the centre.
Practice twirling the wheel/stick between your fingers, so that it rotates but doesn't change position. It may help to rest the other end of the stick on your desk/table.
Take a fine black marker pen, apply it to the (slowly) twirling wheel next to the hub.
When the ink is dry, you can now paint the rest of the tyre without getting black/dark grey or whatever on the hub.
Result, round looking wheels.
When painting the hub, it may help to thin the paint slightly. This will stop it hiding detail, and it will flow/conform to the outside of the hub if you go onto the tyre.
Pete