After a bit of testing it turns out the the 2-player game doesn’t work very well as written. Because the players’ block pools are symmetric with each other, it’s too easy for them to just mirror one another’s play, making for quite a dull safety game. And I suspect this might be true to some extent with the 3-player version as well.
I think we can get around this by introducing more randomness in the setup, which will also have the advantage of requiring less of the fiddly block-counting. So now, the game consists of 72 blocks, 12 of each colour. At the start of the game, each player takes 12 blocks at random from these 72, to form their personal pool; then picks one of these at random to be the base of their tower.
I’m also going to adopt Dom’s scoring suggestion, whereby instead of surviving players all scoring according to the height of the best untoppled tower, instead they each score according to how many blocks they have left in their personal pool. This gives a slight scoring advantage to later players in the round, but it’ll even out over a series of games, and it’ll be helpful for breaking ties if there’s more than one player who’s good enough never to knock over a tower.
This evening I hope to be testing the several-players version: it’ll be interesting to see if that adds any depth of interest to the 2- or 3-player game.
Then the next challenge might be to come up with a plausible theme for it…