A prototype display developed by Opalux.

Last week, a panel of experts at EmTech@MIT discussed technologies that could hasten the arrival of color e-readers.

While the panelists agreed that high-quality color displays could make portable reading devices more attractive to advertisers and deliver a richer experience for readers, they were less unanimous on the best way to deliver color screens.

Two companies are hoping to use reflective microstructures--the same kind seen in opals and on butterflies' wings--to develop color displays.

Opalux uses a sponge-like polymer structure that mimics that of an opal. When a voltage is applied, the material expands, changing the wavelength of light that it reflects."So you can basically take one material and get all the colors you want," says the company's CEO, Andre Arsenault.

Qualcomm is also making color displays with photonic microstructures. The company has developed a MEMs structure that sits on glass and opens and closes depending on the voltage applied, imitating the way gaps on the surface of a butterfly's wings allow certain wavelengths of light to reflect back.

Achieving high quality shades of black, white and gray remains a challenge for such screens. And, just like a stone sparkling at a certain angle of light, the color can sometimes change when viewed from different angles.

Another company, Kent Displays, has developed a technology that reflects different colors using three colored layers of liquid crystals placed on top of glass or plastic LCDs. The company has so far made thin, flexible displays that consume little power, says CTO Asad Khan.

E-Ink, which makes the displays for Amazon's Kindle, uses micro-encapsulated charged particles that move in response to an electric field. In 2010, the company plans to put a color filter over the electronic paper to add color. However, E-Ink's product director Lawrence Schwartz says that the industry needs to make sure the devices are low cost and low power and are usable in direct sunlight.