On that dark empire sourcebook cover.

The color difference is probably an accident either in the printing or who ever did that digital scan of the book. It is probably the second option. I don't think its a deliberate creative decision, anymore than Vader's Saber turning purple or pink, or Maul's saber turning up orange on the blu rays.

It could just be a continuity mistake like the revenge of the jedi teaser poster, the star wars marvel comic issue #1 or that book where Luke gives Leia a sith red lightsaber as a present.

Or as you imagine it also could be on purpose but it hardly matters because it is legends not Canon it does not need a retcon to explain.

Luke never fought Sedriss the Emperor never was reborn.