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I think you could get this down to the size of one of those four color pens, but the idea is three tiny stepper motors tension springs with small load cells that feed three primary colored crayons (RBG or CMY) down to a tiny heating coil so that you could draw in any color. USB would power and control
the pen, which changes color by varying the pressure applied to each of the crayons to vary the mixture of the wax finally applied. The load cells provide feedback on the tension of the springs.
I'm not thrilled with the need for a wire coming off the back so the "executive" version is rechargeable with a bluetooth connection, though I doubt that many executives use crayons, but you never know.
AnyColor Pen with ColorPick
[xaviergisz, Jun 24 2008]
Wireless USB [Wrongfellow, Jun 24 2008]
||I think the USB only encumbers it. Could control the colors with a slider on the pen.
||You'd want to use CMYK rather than RGB as you'd be using subtractive rather than additive colour.
||Also, with WUSB (link) you wouldn't need a wire coming off it, though you'd have to find another way of powering it.