Finished the other two grey/white sections. Yay.
Am now leaning towards a kind of bracket on point for the coloured bits. Like this: } but curvy, and horizontal.
It would work nicely in the corners, halves, and the full centre section. I could echo it and add wobbles on one side and maybe swirls? pebbles? somethings? on the other side. That *might* happen tomorrow, or likely next week.