According to this article it's been around a while and the writer expected to see it by 2010. I can think of any number of reasons they didn't. Collision repair would be one and maybe the legal thing that the cops don't want it, could be a real can-o-worms for them.
Cut and pasted from elsewhere:
Back in 2007, motoring publications reported that Nissan had developed a process for applying a polymer coating (referred to as 'paramagnetic' paint) to automobile exteriors that could instantly change the perceived color of the car to any desired hue through the application of electrical current:
One of the hardest decisions when picking a new car is choosing the right color but the day when cars will be available in multiple colors could be here sooner than you think. Scientists have developed a new coating called 'paramagnetic' paint that has the ability to change colors at the touch of a button. One carmaker looking into the technology is Nissan, which has already developed a self-healing paint.
Before the vehicle is painted,
a special polymer containing the special 'paramagnetic' iron oxide particles is applied to the car’s body. An applied electric current then adjusts the spacing of small crystals within the iron oxide particles and therefore affects their ability to reflect light and change color.
The process is perfect for metal objects like cars because a continuous small current is needed to maintain the desired color. When the vehicle is switched off, the car returns to a default color of white.
The coating has the ability to reproduce any color visible to the human eye and it takes less than a second to change the entire car. The first commercial applications could be on the market as early as 2010.
As far as we know, neither Nissan nor any other major automobile manufacturer has yet offered 'paramagnetic' paint as an available feature on a production-line vehicle, but the days of color-changing cars like the one shown in this video may not be so far off.
Last updated: 13 January 2014
Read more at
http://www.snopes.com/photos/technology/paramagneticpaint.asp#sgdkGV86qiRHGbM7.99