It's even worse than that! Adjectives in general take on special endings. See http://germanforenglishspeakers.com/adjectives/adjective-declensions/.
(Predicative adjectives don't take endings. That's why we say "Der Saft ist rot," and NOT "Der Saft ist roter.")
There are red juices out there. Some grape juices are red, I believe tomato juice is red, cranberry berry juice is red, and raspberry juice is red. All that's missing is some context. A context could be someone trying to explain what juice they drank, but they weren't sure on what it was named.
I think this was more just a demonstration though on how the endings change.