The colours used in this picture are great and i love the crazy nature of the image drawn.
Using light graffiti on an existing object looks great but I think it looks hard to line up in the right place. With the light not lining up with the actual strings of the guitar this image looks a bit messy, however it does add to the effect of the whole photo somehow.
The images below are taken by someone that i know who i will learn how to do light graffiti from.
I plan to experiment with my own light drawing asap.