Thanks for the feedback, everyone! OK, how bout this?
Using "lightslategray" for the untaken spots. Also, the board is now squared off so the dots are in a grid, and the preview piece graphic is blended towards white instead of black. Whatcha think?
Multiple board sizes are quite doable, btw. Should I do the full 7x6 to 10x6 range of boards, or maybe just start with 7x6 and wait and see which sizes are really needed/wanted? Thoughts?
Here's what 10x7 (the biggest) looks like at the moment: