I've been contemplating this, and I'm almost entirely sure it's possible to break the game by staying on the first level. To demonstrate, I simplified the game by imagining a no next level on every square greyed, and a 2x2 grid with 2 colors. On this board, it's trivial to show that you can play forever. If the board is changed to 3 high, it's also trivial to show that you can continue to clear 4 colors. This is done by having the upper and lower pair have different tiles, with the middle pair a clearing ground.
For a minute, let's go back to the 2x2 with no next level. Can you play forever with 3 colors? You can, so long as you know what the next piece is going to be. By knowing, you can be sure to leave that one uncovered for clearing. Of course, you can't play forever with 4, as pure bad luck will have you with no more pairs to make.
Back to the 2x3. With this, our current lower and upper limits are 4 and 6. With the one lookahead, could we play forever with 5? Yes, although it will involve a change in strategy. Have the bottom 2 slots be filled with the first two different tiles that come along. For the third and fourth different tiles, have one on the other two rows, clearing as capable. Worst case, there are one of each of 4 of the 5 tiles, 2 on the bottom and one on the other two rows. from this, if the tile you next have to use is the 5th, you can decide where to put it such that on the next turn you can clear a pair.
I'm not sure how to use this strategy when you have to worry about the clearing rule, though. I'll think about it more tomorrow.
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In other news,
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