Electronic ink is going colour - but you should probably wait a bit for your rainbow-friendly e-reader.

A year ago we looked at e-ink technologies, and suggested it would be 2011 before we saw colour e-ink devices on the shelves. On this occasion it seems we got it about right; though if you can wait until the latter part of the year, and are prepared to pay for it, then you'll get video as well, as e-ink and traditional technologies get closer together in good ways and bad.

On Tuesday E-Ink Corporation demonstrated its Triton colour screen technology, as well as announcing the first device to use it - from Hanvon and priced at around $440 for a 10-inch colour screen that only consumes power while its being updated. But Qualcomm would have us wait until January for the first product using its own technology, despite demonstrating it more than a year ago.

E-ink screens rely on reflected light, so work better in bright sunlight than under the covers; most of them can't be back-lit even if it were desirable, so a reading lamp is essential. They are also static - once an image is displayed it remains, as opposed to the constantly-updating LCD or CRT technology that can make even modern screens uncomfortable to use for long periods.

For an electronic ink technology, the most important factor is how much light the screen can reflect, or absorb on the black bits, and even modern electronic ink tends to be grey-and-slightly-darker-grey rather than black-and-white. The speed with which the screen can update is also important for usability, and overly-reflective glass can affect the users' experience too, but it is colour that everyone has been waiting for.

E-ink isn't just a technology, it's also the name of the E-Ink Corporation. Their greyscale screens already adorn e-book readers from just about everyone.

The latest incarnation, the E Ink Pearl, can be seen offering much better contrast in Amazon's Kindle 3, as well as Sony's latest generation of readers, though it still works on the same principle as earlier designs: tiny beads that are raised or lowered within a pixel to make white or black dots (or slightly raised to make grey):

The beads are suspended in goo, driven to the top by an electrostatic charge

Adding colour to that process is difficult, but can be done by putting different coloured filters on top of adjacent pixels. Make the pixels small enough and the human eye will blur them together to make colours defined by the relative brightness of the sub-pixels, which is what the E Ink Corporation's latest "Triton" screen that was demonstrated last week does:

The pixels are so small that they blur together

Obviously this reduces one's resolution by four, but that might be an acceptable price to pay for colour, which is hard to achieve any other way.