Rare, desireable, valuable are three terms that can be inclusive or exclusive of each other in any item. Many people, buyers and sellers, confuse and meld the three together.
I'd say rare means not many made. As others have said, does rare equal desireable or valuable, certainly not. Inherently any E body is more rare than any mustang or camaro. Considering that the entire 70-74 production run of Challengers and Barracudas combined do not equal any single year of Camaro or Mustang production and suddenly a mopar becomes a very rare car, yet there are Camaros and Mustangs that sell for more than some Challengers and Cudas. Rarity is not a gaurentee of value.
Now, should colors, options, trim, etc be a part of the consideration of rare, maybe. Certainly if you slice it that way, it is possible to really narrow down the rarity of a vehicle to a one of just a few. Even within that there is desireability that can influence value. This is the gray area that allows knowledgable buyers, and sellers, to nit pick each other apart when it comes down to determining desireability and as a result, value. I think when you take all the possible combinations of all the available standard and option features on any set of vehicles, the number can be staggering about how "rare" a certain combo can, but assembly lines were not designed to produce exclusively rare cars, so a certian amount of sameness is inevitible to allow production to run at a rate that allowed the vehicels to be affordable.
You also have the just awful combos out there that are rare, but want to make you gag. Like a yellow vinyl top on a frost green car with a tan interior. Rare, yes, possible, certainly, but would anyone really want to be seen driving it? In this case, a less rare black on black car, regardless of trim and options, would be more valuable.
