Sadly, the problem with registration is you're not dealing with the original art, you're several steps removed. You're taking a comic page that's out of register printed in CMYK (Cyan, Magenta, Yellow, blacK). But, on the computer these colors have been converted to RGB (Red, Green, Blue). Now, you'd think you could just switch it back to CMYK, select and nudge the out-of-register channels back into register, but if only it was that simple. Not even counting what aging does to the colors, the conversion from one color mode isn't 1:1. After all, it has gone from 4 channels to 3 channels then back to 4. There's color shifts and computer calculations on how to create the blacks, how dense do you want your colors for the printing press, etc. It'd be a miracle that you'd end up with the exact same separation they had and only had to nudge the conflicting channel back into play even though you can clearly see that the red is out of register one way, the yellow another.
Another unfortunate bit, my way of recoloring does a good job and would look wonderful for printing (if the images were high enough resolution). However, on the web, it does make the pages look a bit fake, the colors a bit TOO perfect and flat (although there's ways around that too if you really want to go through that much trouble, add noise filter or something similar).
Wish I knew of quick and easy ways. But, for me, it's because I've been using Photoshop daily for almost 20 years and have done a few of these on my own in addition to working on thousands and thousands of graphics of all types for work.
