Sorry, I actually meant The Firce Awakens…so many movies…
In ANH he had a silver leg, but probably couldn’t tell until the remastered movies came out.
He got dismantled in ESB and repaired by Chewie, back to normal.
I’m Force Awakens he even mentions it to Han ‘you might not recognise me as I have a red arm’
Only thing I can think is it’s to make up for all the questions about why Obi Wan and Anakon (as Darth) don’t recognise R2 or 3PO in the original movies, even though they’ve known them from the prequels.