Hmm... Maybe I'd bleach and then tie-dye the Ringwraiths' 15 yards of cloaks (each); that'd be hilariously stupid to watch; the Many-coloured Riders chasing after Asfaloth.
Also, I'd dye Saruman's hair black to get a picture of what he once looked like.
How about turning a fire hose on the Durin's Bane?
Additionally, Celeborn's hair was supposed to be silver. The actor portraying him (Martin Csokas) wore a long wig, but his dark hair still showed for inches at the roots of his hair. At least dye it! I mean, Brad Dourif had to shave off his eyebrows, why can't Csokas dye his hair? That's the only one I actually didn't make up, here and now. Though I have wondered what a pic of Curunír 'Lân with black hair would look like.