Good work! This is just like Flood from Simon Tatham’s Puzzles, one of my favorites from that collection :D I like your artwork for the paint buckets, but I would find it easier to just click the colored cell to fill with that color. You could have both options!
Another small bit of feedback: It would make more sense to me if the New Game button was directly below the Size and Colors options, since those are what affect the new game.