I use the circle template method..
Get a circle template from a craft store.. you can get em with about 20 or 30 different size circles on them..
I airbrush the black on the tires/roadwheels first..
after that dries. find the circle on the template that matchs the hub size.. mask off the other ones around it.. hold over the hub and airbrush straight through the circle to spay the hub color..
works great..