I think I have this figured out now. A while ago I made a circular piece with a second circle inside it, that I used as a base for the grails to sit on in the last show. It looks very good, with a soft transition from the central white circle to the darker burnt sienna outer circle. I’ll try this later today on the orb, which I added some orange to earlier. Under a layer of the semi-transparent zinc white I think this will work well.