British and Canadian Scientists have developed a polymer that contains layers that shrink and swell in response to electrical currents.
The material changes colors on demand, and can assume any color of the visible spectrum. It is similar in design to an opal in nature, but the results are obviously more controllable.
According to research team member Ian Manners at the University of Bristol in the United Kingdom, “The polymer is crucial to the whole thing. It contains iron atoms which can exist in two oxidative states. The more you oxidise the system the more it swells.” As the voltage is increased, more iron gets oxidized which results in more swelling, and hence there is a greater shift into the red.
While one use for this technology is full color electronic paper, I think it would be simple enough to tie this into a ring with a power source...a programmable mood ring.