Wikipedia defines thermochromism as "the ability of substance to change color due to a change in temperature. A mood ring is an excellent example of this, but it has many other uses…"
Apparently you can make thermochromic Moleskine covers, too. Sean Michael Ragan did. His notebook is part of the MoleskineAsia "My Moleskine 2.0" exhibit.
There's even instructions how to make your own. Check it out!