Actually, R2 changes colors in this scene.
R2 is blue when shown though the rear window of the cockpit of Luke's X-wing.
(for instance after Luke hears Ben's voice telling him to trust his feelings.)
When shown sideways he is black.
(After Luke says: "Red five standing by", or later when R2 is repairing the x-wing.)
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Now I've taken a quick look on my old pan & scan 4:3 O-OT copy of Star Wars (taped from television a long time ago) and all I can say is R2 used to change color in this scene all along. No changes were made to his color for the 2004 dvd-edition.