there are two ways you can do this.
you can either paint the whole thing enamel black. then with a slightly moistening cotton but in white spirit.enamel thinners remove the paint from the spokes
dont worry, the thinner will not remove the chrome plating.
the other way is to simply appply a black wash to the wheel. this will fill in all the gaps between the spokes and leave the spokes paint free.
try searching the forum for "wash" or try this site out.
Weathering. - Welcome
and welcome to the forum!
Richard