I know they haven't been released to stores yet...
Z6 - They have all colors on display at the Bridgewater NJ store; saw them this past Sunday. Yellow one looks, to me, like the metallic version of a lemon Charms lollipop. Definitely NOT banana yellow. Least favorite color of mine. It looks as if the color itself is transparent, so you see through to the metal. Not opaque like enamel.
Green, orange, red, purple all look good; blue is OK, but I'd like a bit closer to true cobalt. Black and silver are alright if you like that, though I'm not sure how I feel about the black clickwheel on silver.
BTW1 - Now I agree with those complaining about coverflow on the nano; it just seems wasteful without the tactile input of touch screen. Especially if it adversely affects battery life as much as some claim.
BTW2 - Bridgewater store is TI-NEE! Place felt completely log-jammed, then I realized that half the people in there were the 6 or 7 salespeople on duty. I guess I'm spoiled by the SoHo store.