Kerry, I can tell you from personal experience that once washed, the colors of a poster burst forth from the paper. A claim such as "The Little Caesar poster, as presented, now looks better than it probably did when first printed" cannot be proven in any possible way. Also, the paper itself whitens in the cleaning process, brightening the colors further
Don't forget that these posters were essentially "preserved" away from light as each successive poster was applied to the board and so the original saturation levels of the dyes in the ink most probably retained their original pigmentation within the layers and so once washed and brought forth from the dirty layers above show their original brilliance. I also don't think it's necessary for Heritage to comment (if they commented to every sewing circle with a criticism it would reduce their time to do the important work of selling posters), but rather for those who are critical to actually go forth to examine the posters themselves as they are the ones that are curious. Then you can actually say "I visited the poster and this is what I noted......." as opposed to putting forth conjecture as fact.
that said, it is obvious these posters all have some degree of restoration. Part of the restoration process is restoring the colors and certain dyes reflect light differently in photography, appearing brighter. This could yet be another factor in how you see the color.
one more issue: color calibration
Harry noted that a photographer uses a color bar to match pure white under white so as to get the most natural colors. However, the photographer also calibrates his computer monitor to make sure that he sees is as close as possible to what the camera sees. I want to know how many (if any at all) members on this forum have ever calibrated their monitors to this degree?? No need to answer.. it's probably less than 1%
oh and one more thing. Heritage is a corporate entity with may departments under autonomous control on different floors of a huge building. The photographers are not under movie poster control - they are under photography department control. They have certain learnings that have nothing to do with the interests of collectors wanting to see each nook and cranny of wear in a poster. The photographer's job is to make the item being sold look as good as possible with the realm of looking correct. His visual interests are not the same as your visual interests. There is nothing nefarious about it. The photographer is doing the job they were taught to do.
Understanding these facts in concert with each other is the only way you can even make educated guesses about why the images look as they do