I wouldn't sweat the color of the logo. I have noticed that the color on the correct Eastpak logo often appears orangeish against the maroon/red of the backpack itself. The entire film appears very warmly colored and I'm sure all of our old packs spent some time fading in the sun. Impossible to prove one way or another in my opinion but I can guarantee the zipper pulls on Marty's bag are chrome and not black.
I have one of these also with but with black zippers and a padded interior back panel in yellow. What color is yours? Marty's? I have seen grey and turquoise on eBay before.
George Costanza on Seinfeld has a blue one just like Marty's. A popular and durable film production favorite? Probably just a coincidence!