I have a pic of a Mallard Green one from Ford Fair 2009. Reg is K916 ATX and had signed factory build sheet to certify it was the last off the line.

Also have a pic of a Radiant Red Sapphire 4wd from Ford Fair 2006. Reg is K191 PNS and I seem to remember it had some paperwork saying it was original and not a respray. (I had a closer look thinking replica ahoy!) lol

To clear up the blue sapphire F118 KEV above is definitely Crystal Blue. Have seen that car a few times.

Classic/Fast Ford magazine did an article on Cosworth Sierras within the last 12 months I think, and I seem to remember they listed all the factory production figures with colour breakdown. From memory the rarest colours were
RS500 - Moonstone Blue
2wd - Crystal Blue (I think Black may of been SVO?)
4wd - Silver
However I seem to recollect that it did say the figures did not include SVO (Special Vehicle Order) cars hence the Mallard Green & Radiant Red 4wd Sapphires at the end of the run and the Red 3dr in above posting not being listed.

I also think I have a vague memory of reading a magazine article about the Red 2wd Sapphire above (or one like it). Resprayed and rebuilt to standard spec, or built from an un-registered shell that was sprayed Red with new parts as I remember. But I'm not certain on this point so feel free to correct me if you know differently.