This was tougher to implement than I thought! - But, it's now looking like a week or two of work before it's ready (time permitting!), to create leaderboards, modify the level selection to ONLY show puzzle-mode enabled levels, give a relevant endgame summary screen, etc...
It plays well so far, although of course, even with the correctly assigned pieces near the end of your game, it's still possible that you can have already rendered the board uncompletable with an earlier move, so in this mode it's VERY important to take your time and play strategically, carefully studying the board when placing new pieces, to avoid breaking it, etc!
I've also added those additional colours (both for my own levels, and in the Level Creator/Editor!), and re-worked some of the existing levels to be completable in Puzzle Mode, plus some other tweaks and fixes!
Some PUZZLE MODE tips!:-
I'd advise always doing your best to leave yourself somewhere to place a problem piece type that you may get, without causing 'islands' of empty slots that do NOT divide perfectly by 4 (tiles per shape).
PLUS, on tight sections of a level, try to picture if a piece that you're about to place might render other slots unplayable! (ie. if only a LONG piece will fit inside a closed space, try to picture which other pieces need to 'butt up' against it to solve that section without rendering other slots unplayable), then try to keep those slots free for the correct piece, etc...
Once you're down to five last possible moves / 20 free slots left, the computer will step in and try to dynamically allocate the best mix of pieces (not just ones that will fit for the current turn, but more ones that will fit the MOST pieces into how final free slots) to allow you to have a better chance of completing the puzzle, but of course, HOW you place those pieces plays a big part in that... ;)
Of course, there will be some instances where you are unfortunately given a none-optimal piece before this point (for example, if you've created a very tight, scattered play board), but you can either retry the level, or if you're playing on an easier difficulty setting, you will have access to a limited number of 'SWAP's to request another (always different shape, if it's possible) piece.... :)
NOTE! - SWAPPING will penalise you time-wise, and will not count as a 'flawless' puzzle completion!
...ie. You will 'beat' the level, and unlock more levels, but will NOT gain an online ranking - so if you have to use it extensively, you'll probably have to kiss that Leaderboard 'Fast Completion' ranking chance, and flawless puzzle completion achievement, goodbye!