That's what I was taught in school, at any rate. As long as it is being used as a name. Superman would be The Man in Blue. But you could write:
The man in blue crossed his arms as Lois took out her notepad.
"Who are you?" she asked.
"My name is Kal-El," the man replied.
See? Here, "the man in blue" is just an identifier, not a name. Though I have seen identifiers sometimes capitalized as well, especially if that is the *only* way a character is referred to in a story. So, if Superman/Kal/Clark is never used, as a name, and he is only called "the man in blue," then I would write it as "The Man in Blue."
Know what I mean?
