Heh, for a mess it's not bad. The main thing you might want to play around with though is getting a better flow with your colors, the third picture does that well but the first and the second both show a lesser migration in your colors. Graffiti is an art that especially builds a lot on blending your colors well together for your intented motive.

I don't have any hints on how to work with graffiti in general but I think you might want to watch some of Bob Ross' videos which are not only very interesting but also teaches you a lot about using traditional colors.

http://www.youtube.com/user/ilium86