Really interesting entry. It reminds me a little bit of 2048.
Like other comments say, the onboarding could be a little better (perhaps have a small number in the corner of each tile for its level? The green borders for upgradeable tiles are a nice touch).
The graphics are simple, but polished. The "age" meters look nice and the typeface matches the aesthetic of the game. A little extra sounds, effects, and feedback on various events (e.g. a tile dying) could push it to the next level.
My high score didn't seem to be written to, but I got as far as having two pink tiles on the board (and then promptly lost because I had neglected the rest of them... oops.
Good work for 72 hours!