If you have a router or a rotary tool with a base, that could be routed into the disc, then filled with colored epoxy or self-hardening clay. Just make a template to do the routing, since you have 40 of them to do. To do the routing safely, you could lay out the discs on a board, then rout them all together. After the filling hardens, sand everything flat and cut out the individual discs.
Tom