The problem with this concept is that a person's skin is rarely the same color from head to toe. I mean, take a look at yourself: is your butt the exact same shade of fuchsia (or whatever) as the back of your neck?

A better approach is to snap a full-scale photo of the patient's nude body and print the result onto a perfectly form-fitting gown, with optional holes for eyes/nose/mouth/unmentionables. That way you can monitor changes over the entire surface of the patient's skin while keeping his dignity intact.