Welcome back, Hypercolor. This heat-sensitive iPhone 4/4s backing responds to the touch of a hand--or lips or other body parts traditionally reserved for photocopiers--with thermochromic color changes that mirror its shapely outline. The self-adhesive skin starts out black (cold) and morphs into red, yellow, green, and deep blue as applied temperatures increase. A range of 90 to 95 degrees F, just below body temperature, produces a slick spectrum of color change as the surface warms and then cools again, as well as a multi-hued gradient emanating from the core of the heat source.
Backings are only 0.005" thick, so they won't add any noticeable heft to a phone, and will also fit comfortably under installed bumpers.