In this case I cut cirlcles of masking tape out using a compass cutter and stuck them on the middle, masking like this is ok since the middle is the natural colour of the plastic. The black is just rattle can hobby enamel.
To get the insides neat and coloured (say silver which is common) you can shove the wheel hard onto a cocktail stick or round tooth pick (both of which are tapered) then spin it round in a very slow moving drill held in a vice whilst holding the brush still.......