Translation:The blue one or the red one?
I put "the blue or the red one" In English, I might say it this way or the way duo translated it. Dick: I'm going to buy one of these shirts. Jane: Do you want the blue or the red one? So I think my translation should be accepted.
Where does the "one" come from? Which color do you want? "The blue or the red?"
Think of it this way: Which color is (the one that) you want? In English, the word "one" is understood, whether or not it's used is optional.