I have just touched mine up with an artists small paint brush (cotton bud will work too) whilst on the car with a can of silver caliper paint I had lying around. Other than that you can get plastic covers off fleabay etc....or go the full hog & take them off, wire brush them(one of the circular type that goes in a drill does a really good job), wipe down with thinners or simaler, get them nice & warm so the paint adheres well (stick them in the oven or hot air gun, hair drier) & finally paint them. Hammerite spray silver or black will do the job & a top tip rest the wheel bolts or nuts in the top of an empty wine bottle. If you dont have the equipment or space to remove all the wheels in one go take a few bolts/nuts of each wheel at a time & don't use the car it would allow plenty of over night drying time before you put the freshly painted ones back on.
There you go just a trip out to get some paint & 20 bottles of wine & your ready to go
