I would consider Turquoise Blue, Jade Green, Zanzabar and Orient Red rare. In some cases, it can dimish the value, as your pool of potential buyers is reduced, as opposed to Arctic Silver, Guards Red and Black.
It's all in the eye of the beholder
If I were to buy a silver Boxster, I would look for Polar Silver, but I don't know if I would pay a premium. There are definitely some combos that could seriously diminish the value, Speed Yellow w/ Boxster Red interior comes to mind along with any thing with Sea Foam Green interior.
I would also advise against gold plating:
