Superparamagnetic Colloidal Photonic Crystals... apparently.

From the Department of the Very Cool: scientists were able to change the color of the iron oxide solutions simply by changing the strength of the magnetic fields the solutions are in. The analogy is like tuning a TV (the old CRT tube TVs, not the new flatscreen ones). The researchers studied very small particles (nanoparticles) of iron oxide suspended in water, much like milk or paint are both suspensions of substances in water.

When the strength of the magnetic field is changed, it alters the arrangement of the spherical iron oxide particles in solution, making them move closer or farther away from each other. This changes how light falling on the particles passes through or is deflected by the solution which gives color. Because they affect light they are called photonic crystals.

The new technology can be used to make an inexpensive color displays by forming millions of small pixels using the photonic crystals. A different color for each pixel can be assigned using a magnetic field.

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