I would guess they were working off black and white stills as happened with the SW adaptation, so nobody knew what the actual colors were? How else can one explain purple Yoda?
On the flip side of the coin, I vaguely recall some Gold Key Star Trek comics with wrong uniform colors in the mid 70’s, when finding out what the proper colors were should have been as easy a catching a TOS rerun. 😉
That makes sense - the TESB stuff, not the Star Trek (although I guess colour TVs were quite rare back then). I also love the shots of Vader on the bridge of his Star Destroyer with only that single window visible - clearly the photo reference lacked the later details of the matte painting.
One of my favourite Marvel adaptations was the Close Encounters one, where they had no photographic reference at all and no permission to use actor likenesses! I think it’s pretty amazing what was achieved under those circumstances!