It's a good game, it's a bit short though. I think as a simple game it's pretty easy to solve the puzzles, and while you can make it more complex by introducing a larger grid and additional colors, it will ultimately be the same problem. Adding 20 more levels at the same level of complexity would make the game seem trivial after a while.
It might benefit from additional types of tiles that introduce complexity, maybe like a tile that reacts to the color placed next to it and becomes colored the same way. Maybe a tile that reacts to all the colors next to it and becomes a mix. (IE: You need a purple tile in one spot, so in order to have that tile or tile group to turn purple, you need red and blue squares next to it). Sparse use of elements like that keep the problem from being identical.
Then again, maybe it's not needed. People enjoy working through books of sudoku problems.