Humidity sensitive paint

check here for starters.

http://en.wikipedia.org/wiki/Humidity_indicator_card

You can also get the type that once exposed to water or high humidity, do not change back. This site can zero you in on what you might search under such as specific chemicals.

This is pretty cool!

http://www.coolthings.com/the-mini-chameleon-%E2%80%93-world%E2%80%99s-first-color-changing-car/

Thank you "Charsley99"

This was informative :

What I need is a reversable paint to make a funktion as described in my request.