You can make/carve the guitar body using normal batter, but make the neck in multiple colors (click on the link and scroll down to the third photo...it has 28 layers!).
You could still add edible photos to the top surface, but this method would allow many, many layers without having to worry about vertical stacking issues. And limiting the "multi-colored" layers to only the neck area means you won't have to bake so much. Just divide the pan or use disposable mini-pans.
http://alfrescofoodandlifestyle.blogspot.com/2012/02/happiest-rainbow-cakesever.html
You could still add edible photos to the top surface, but this method would allow many, many layers without having to worry about vertical stacking issues. And limiting the "multi-colored" layers to only the neck area means you won't have to bake so much. Just divide the pan or use disposable mini-pans.
http://alfrescofoodandlifestyle.blogspot.com/2012/02/happiest-rainbow-cakesever.html