Thanks for the feedback!
Yeah, I agree it could be bit more lenient - I had trouble coming up with a rule that would work in all situations. In the end I decided that you have to match *all* surrounding dice was easiest to reason about - if you just have to match one, the game actually becomes *easier* as it progresses :|
(btw, in your example, you could put a GREEN 4 in that slot, since it would match the right dice by color and the top / bottom by value.)