Red Oni, Blue Oni/Theatre

Vladimir and Estragon in Waiting for Godot
In Rosencrantz and Guildenstern Are Dead, Rosencrantz is red and Guildenstern is blue.
Hamlet (red) and Horatio (blue), conforming to the Zeroth Law of Trope Examples.
In the movie version of the musical Jekyll and Hyde (yes, the one with David Hasselhoff), Dr. Jekyll is shown with a red light (hard to tell in this picture, sorry) and Hyde is shown with a blue light shining on him
Romeo and Juliet gives us Romeo (red) and Benvolio (blue), Mercutio (red) and Benvolio (blue), and even Romeo (red) and Juliet (blue).
To a degree, Henry Higgins (red) and Colonel Pickering (blue) from Pygmalion and My Fair Lady.