Don't like the colour of your new car? Flip a switch and the lurid purple paint job can change to a classy tangerine.

That's the promise of a new type of colour-changing pigment that could revolutionise signage, posters and interior decoration.

Scientists at the University of California have fabricated microscopic polymer beads that change colour instantly and reversibly in the presence of a magnetic field.

Nano paint

The researchers embedded arrays of magnetic iron oxide nanostructures within a polymer microsphere, enabling its colours to be switched between hues by changing the microsphere's orientation with a weak magnetic field. The new system has the advantage of producing bistable colour states, required for making rewritable displays.

The vivid colour in the new materials is caused by interference effects rather than pigments, very much like colourful bird feathers and butterfly wings. The technology is also extremely fast to switch on and off, making it suitable for large displays such as advertising hoardings.

Applications of the new material include display type units such as rewritable or reusable signage, posters, papers and labels, and magnetically-activated security features. The new material can also be used to make environmentally-friendly pigments for paints and cosmetics, as well as ink materials for colour printing.