I have over a hundred of those cels myself (from all 3 films). I've been collecting them since Willitts Designs put them out in 1995. You can still find plenty of them on eBay- no two are alike. They were made from 70mm interpositives. The SW ones are pretty plentiful, followed by ESB. They made far fewer of the ROTJ ones, and good scenes from that film are harder to find. I think they're one of the best Star Wars collectibles that have ever been made. I had to stop collecting them because I was starting to get a little nuts with them...LOL!

They're probably the best color reference you're going to find, as far as what the original films looked like.