Neat idea, and works well! Only problem: red on green is quite hard to see for people with deuteranopia.
Definitely with you on that one. In the limited amount of time I had, I hoped to at least make them varying intensities, but didn't get a chance to check if they were far enough apart. When I do get back to tinkering with this again, piece visibility (in all cases) is definitely at the top of my list. (Ten minutes adding X's and O's to the blocks would be a decent temporary solution.)