What about using photos from the set as reference?
The book The Making of the Empire Strikes Back there are a lot of set photos that are probably fairly accurate. If you know the color of certain objects in the film, clothes, skin, Yoda, the ship you could try and make those colors match. If you take more neutral objects that have greys, whites and blacks in them it will help you balance the shots.
To me Empire is probably the hardest film to fix. Star Wars luckily has a few Technicolor prints. With Jedi the film stock finally changed so those prints probably have less faded color. But what about Empire? It's kind of stuck in between the eras where print stocks were at their poorest.