Aircraft wheel masking / painting;

Visit the local office supply or craft store and pick yourself up a circle template guide. You find the opening that best fits the center wheel section, then mask off the surrounding openings. Then carefully place the opening over the rim and give it a light spray with the color you are applying.

Otherwise you can lay tape over the side of the wheel/tire and cut yourself a mask by going along the edge of the rim. The maskes included in masking sets generally have you paint the rim first, apply the masking disc, then spray the tire color.

I typically spray the rims, let them cure for a good long time then brush paint the tires. Paint in the right consistancy and using the proper brush allows the paint to flow up to the rim and you get good smooth coverage along the side wall of the tire. Then I paint the tread area with a slightly lightened color of what I used to paint the side walls to replicate wear.

You may even find that the humble paper hole punch may yield a suitably sized hole. If you have several punches of different brands handy, try them all, they will be slightly different.

I have on occasion found the right size and punched an index card and used that.

I use two different ways to mask wheels for painting. One, the circle template Gerald mentioned. The second, I use a circle cutter to cut small disks out of Tamiya tape and apply the tape to the painted rims. I normally airbrush all my wheels, the only exception are very tiny wheels like tail wheels or castors.