Lights-Out component which makes things very tricky.
I solved the first batch of levels pretty quickly. Then the 3 and 4 color problems showed up. That's when I realized how interesting and complicated this game is. In cohex if you've lost your last green tile, you can never get it back. This game is really for serious puzzlers only.
There are 77 levels and I've done about 20.
Update: I had to do them all!