Allright!!!! I finally figured out why the colours on my monitor were wrong.
Thanks to KiwiJochen who correctly insisted it was a cabling problem.

I eventually got a copy of the WDS and checked that the co-ax cables carrying the RGB signal were connected to the right pins in the blue connector (nav) and white connector (monitor). I discovered that sometime before I owned the car somebody must have been doing some work on the system and had transposed the connections for the red and blue signals. I switched the pins and Voila! I have a normal looking screen.

Unfortunately I figured this out after buying a replacement 16:9 monitor....so if anybody wants one, I'm open to offers before it goes on eBay along with my old MK3 nav.