Grey cells are sealed. Empty cells are yours to fill from your hand.
Place a card next to another card. If your touching number is higher, that card flips color — grey becomes green, green becomes grey. This can happen either way as you keep placing.
Win the instant there's no grey left on the board.
You get 3 undos. Each one removes your most recent placement (yellow border) — you can't undo further back while it's still down.