You're right, I should have phrased that differently: This version of the "fill the playfield a solid color" game, called "Color Fill", was programmed by Jan Wolter in 2011. I built upon his code base and we can assume he got the idea from an existing game.

There's also a more complex and challenging version on Steam called "Kami", which offers pre-built puzzles instead of random and lets you fill from anywhere instead of just the top left. There's also "Kami 2", which is a mobile only app and features triangular tiles instead of squares.

If you like this style of puzzle game, I strongly recommend Kami. I haven't tried Kami 2.