Disable any visual feedback if the user clicks and drags over the grid.
Find a way to 'pulse' the color which is in the top left, so the user knows where the buttons apply to AND that the buttons are active and apply to the selection.
Grey out the buttons which do nothing.
Remove the 3,2,1 in practice mode
The colors need work, I'm not color blind, but I struggle with Orange/Red/Yellow here when they're adjacent
Display the effect a button will have when you hover over it on PC, (maybe) when long-pressed on mobile
Space out the initial 'mode selection' buttons a bit more
"I'm colorblind" needs to be bigger, and clicking on the text should affect the state of the checkbox. 48dp touch targets minimum
(maybe) Change "I'm colorblind" checkbox to a 'Colorblind mode' toggle.
(analytics) the game feels is slightly too long. A/B test to see what works. Intuition tells me you'll want 12 moves and a smaller grid, rather than 16.
(analytics) Look into the relation between win rate and retention AND how users interact with the app if they lose their first game. You might want to make the first game easier (in practice mode).
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(more serious investment). Show don't tell. Remove the rules. Turn it into a card-view or similar with animations. I shouldn't need to be able to read to play the game. This has the advantage of making it open to any internet user in the world of any age.
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Leaderboard: I don't care about the leaderboard or points because the points are meaningless and not explained well. I'd cut it.
I'd experiment with "X% solved this in 16 tries" (obviously with handling for the first user)
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(maybe) consider slightly dimming the colors which aren't adjacent to the current 'blob'