Gold is kinda rare -- and it looks really good in person. One of our club members has a gold roadster.
Yellow is rare - expecially with the white/black interior -- but I'm not a big fan of the yellow.
Silver is pretty rare and it looks awesome. The silver allows for most of the color combos to look good, unlike many of the cars which have IMHO - mismatched exterior / interior colors. Gold exterior with red interior is........we'll, I'm not a fan (no offense - just personal taste). Many other colors and combos don't work either.
Machine grey is awesome, very rare and if I could find a reasonable price on one with the oyster interior......we'll, I'd probably have to buy it.
Red isn't that rare, but a red roadster with the oyster interior is my target second XFire. I think it is a great combo.
Aero blue has grown on me and either the oyster, black or grey / or combo grey is great in this car. I didn't like it at first, but I'd consider buying one in this color now.
Oyster White - not so rare but look great when clean. Almost any color combo interior works well.

Not rare are the SSB's (my color) but I happen to love it's camelian look as it changes in different light - direct sun, evening, inside a garage, etc. Also, a classy interior if it's the slate / blue (my color). I don't like the red or straight grey or black nearly as much.
Black isnt' rare and while I love the car in black, I've had black cars before and it's great clean, bad dirty -- which means even more washing that I do now (which is alot). I like clean cars.

Eitherway, I'd take any of them because I love these cars.