Rules governing "quite a [adjective]" word order

The word quite describes the degree of intensity to which the adjective modifies the noun. If I say that something is quite big, then it should be understood to mean that the thing is bigger than many things but not as big as it could be. It might be less common to discuss varying degrees of purple, but I don't think I would call it ungrammatical to do so. I would say that the answer to your first question is that you can say 'quite a(n) x' any time the adjective can have its intensity modified. I can't think of an example where I could say 'a very x' and not 'quite a(n) x' (assuming very to mean 'to a great degree' and not its alternate meaning).