The BlinkM would most likely be a better choice. Mainly because it does use a microcontroller, and you could create custom firmware and reprogram that controller to mix colors based on the pulse-width servo input. The BlinkM and the ShiftBrite won't work with your application out of the box, but the BlinkM could be reprogrammed to do so. You'd need to learn firmware development for the AVR microcontroller.
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The BlinkM would most likely
The BlinkM would most likely be a better choice. Mainly because it does use a microcontroller, and you could create custom firmware and reprogram that controller to mix colors based on the pulse-width servo input. The BlinkM and the ShiftBrite won't work with your application out of the box, but the BlinkM could be reprogrammed to do so. You'd need to learn firmware development for the AVR microcontroller.