Брюки цвета кофе is natural in Russian. Pretentious, but still natural.
"Брюки цвета кофе" ("pants of coffee color") is natural, "коричневые брюки" ("brown pants") is natural, but "кофейные брюки" is not. In fact the latter would likely be interpreted as "coffee pants" or "pants made out of coffee."
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"Брюки цвета кофе" ("pants of coffee color") is natural, "коричневые брюки" ("brown pants") is natural, but "кофейные брюки" is not. In fact the latter would likely be interpreted as "coffee pants" or "pants made out of coffee."