There are many different paint technologies that allow the appearances of color changeability. Some technologies rely on temperature changes, others use moisture changes, but most are not durable. BASF now has one that changes with the intensity of light.

http://www.halfbakery.com/idea/Temperature_20Sensitive_20Paint
http://www.alsacorp.com/products/xposurepaint/xposurepaint_prodinfo.ht​m
http://www.halfbakery.com/idea/Color_20Changing_20Car

Pearlescent automotive paints and various metallic and metalflake paints have been used on cars for decades. But what you may be referring to are the more common "chameleon" urethanes of recent years, such as these ...

http://www.kustomrides.com/chameleon.htm

I have a Lincoln towncar I thought was gray when I bought it, but discovered on the first really sunny day that it is actually a subtle lavender.

Same sort of experience...but with a Honda Civic hybrid. On the lot on a cloudy day it looked silver. Next day when the sun came out it was clearly blueish.

Funny, I found the color I'm looking for, but is lavender really what it's called?
http://www.rodster.com/assets/Crabtreerod2ax.jpg

LOL

Changed, that is better known as OXIDATION! ;-)

Chris: the car in the photo is definitely not lavender. Lavender is a very blue purple in a very light saturation so it is very subtle. Scott D: the unexposed surfaces such as the insides of the doors and underside of the trunk are not sun-faded, and show the lavender much more clearly. Rest assured, it is not oxidation. The few areas on this well-preserved specimen that have suffered oxidation did not turn more lavender, they turned more brownish-red.

There's a product called Mirage Duplicolor paint. Here's their website:
http://www.duplicolor.com/products/mirage.html

Mother of pearl makes it appear different colors when the light hits it.