I have seen some A6's with a color display in the center instrument panel, and some with the monochrome red. What determines which one is on the car?

In the UK, it is either an option on its own, or comes as part of the MMI+ (colour satnav, etc) package.

I've got it on my A6 Avant and it is brilliant - the satnav repeater is really clear it tones in well with the dials. Definitely recommended!

For the 2006's it comes with the Tech pkg. Not sure about the 05's!