The ball was white/light gray. It has the surface texture of plaster of Paris, but it is somewhat lighter than would be appropriate for its canteloupe-like size.
I don't think I actually pictured a whole person as pushing the ball, more likely it was a disembodied hand or the general sensation of pushing it myself.
I remember being specifically intrigued that I pictured the ball rolling back towards the center of the table and pondering why I had chosen the table to be slightly concave. I don't remember more attributes of the table, but I have the feeling that has more to do with inattention to its details rather than not picturing them at the time.
I imagine that, based on the framing of the story, my interpretation was to picture the sphere as a literal entity, but the person as the "concept of a push"... The table probably lied somewhere in the middle.