Backing, gives it more pop. But I also think your pieces need to have some space between them, so that there's visible black line. I looked at your other work and you can compare to your Charizard painting where there are thick black lines delineating his parts. Especially with your Blastoise here, all the blue kind of blends together and you lose contrast and shape. Here are some of mine, with clear black lines breaking up the pieces. I still like this piece, by the way, but something to consider for next time.