Yeah came here to say this. The different states are almost indistinguishable. I also didn't understand that they where supposed to be holes. I'd make the shadow effect much more subtle and have the holes in the background color so they seem like cut out of the red board vs very bright white pieces. Also the selected indicator would be more noticeable in an entirely different color like yellow.

If you want to make it really cool implement drag and drop. When you start dragging a piece add a sublte hightlight to the allowed destination holes.