Dave,
i will pass along a tip Frosty gave me:
paint the entire wheel flat black first.
then use a plastic circle template (from stationary or art store) to mask the tires while you spray the centers grey, dark yellow or whatever.
i just did this the other night on my PzKpfw IV Ausf. D and it worked pretty well.
but if you have to buy a template, i would recommend getting both English and metric circles. the one i had was English (fractions of inch) and the wheel was right between 2 sizes of hole, if you know what i mean.
i put cheap masking tape and paper over the unused part of the template so it would not end up w/ paint all over it or spraying thru the extra holes.
never the less, i ended up w/ minimal overspray, which i decided the wash would hide. i could have touched up w/ brush, but i am lazy, and i don't know if i would have fixed the problem or made a new one!
i will add that i did both sides in succession, painting the backs w/ the face against the table and then putting a piece of sprue in the hole as a holder and pushing it into a foam block. doing the fronts is not fun, as you have to hold the template in place firmly without driving the wet back into the foam! i tried it w/ the wheel in both orientations (face horizontal & vertical), neither was very 'easy' to keep template in place.
all in all i was quite pleased, tho a better fitting template would have made it easier. in reflection, if i had a cheap, spare template, i might have used a bit of sandpaper to make the 'small' hole fit better.
you'll want to put a rubber glove (i use disposable latex gloves) on the hand you use to hold down the template. i had almost as much paint on those fingers as the wheels. :-)
sorry so long.
when i get my pictures hosted, i have some photos that shows what i am talking abt.