Condom or Trojan can be used as either a noun or an adjective. It depends on what function you assign to them in the sentence you construct. I could say, "The kid is a condom because he is a condom head." In the first instance, "condom" functions as a noun because it renames the kid. In the second instance, it functions as an adjective because it describes the kid. Nouns name things and adjectives describe things. In form, both condom and Trojan are nouns when they stand alone. But function (How a word is used.) always trumps form.
"The Trojan looked at the Trojan horse." One "Trojan" here is an adjective and the other is a noun. Think function rather than form and it is easy to tell. :hi: