The short explanation would be, no one is really sure what the original colors were, at least not exactly. These colors are best guesses, based off of the best references available (for v2.5 it turned out that reference wasn’t adjusted correctly, v2.7 tries to adjust closer to what may have been seen theatrically in 1977).
Overall, the Despecialized editions are currently the closest we have to proper HD theatrical versions. They may not be exact, but they’re darn good.