Thanks folks

The fabric spoke cover is a separate component and is only dry fitted here - that means you get a perfect demarkation between tyre and hub. On the inner face however it is necessary to mask, here the circle cutter comes into its own

I had considered cutting out circle templates to mask the outer ring for the whitewall, but there is little extra thickness of plastic on which to mount the mask, as well as difficulties centering it. Alternatively, mask the hub and spray the whole tyre grey first, then mask around the rim with thin strips of tape?
I did mask the inside of the wheel when painting the tyres so that the black paint inside the hub can still be seen through the inspection hole. I am not sure if the centre of the outer hub and the edges including the inspection hole edge would have been metal - I may touch these up with some metalcote.