jmat is giving a correct response. In your reply to jmat, "rich" is not an adjective, but rather a noun. It means "the rich people," and if you used it in conjunction with "people" like I did, then it is an adjective that modifies "people." However; if you omit "people," then "rich" becomes a noun, and is actually the subject of the clause "the rich should pay higher taxes."
Does that make sense?