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Zep, my spa lights have one lens that is red and I HATE it because it does look like blood in the tub...I think the red lens would do the job and not risk staining anything in your tub. Now Drew will probably suggest a good place to get mangled body parts ...they always add a nice flair!
body parts are an interesting idea, just be sure they're not small enough to get wedged in any jets, etc. Good luck, take pics for us!!