have the game end when all the correct squares are colored in. I don't want to have to x out all the other squares

It's only one extra click if you press the right mouse button on the diagonal arrow after filling all the black squares. However I agree that this is not obvious and I need to somehow explain this (and the fact that all squares should be filled) to users. Perhaps it would be simpler to judge completion on the black squares only. I'll look into it.