Green is a good colour for the cross had you thought of also black to go with this for some shadow and white black for skeleton ..Is it a traditional Celtic Cross you would be wanting?
regards
Carol

I received a question about this so want to provide clarification. The Celtic cross part of the guitar could be a Celtic knot instead. That may be preferable since it's not a Christian band. You could even put a bowler black hat on the skeleton, with a shamrock in the hat band. Get creative!