We have a large archive of the film used to print the original comics, and the proofs of said comics as well. It’s not absolutely 100% complete, but it goes back to the 1940s.
Everything that we reprint in TPB sources from that film (or digital data in the case of more recent comics). The older stuff is recolored using the original books as a guide, so that you no longer have the dot patterns of the old limited color screens. And those new files are all digital–so they get added to our digital archives, and are used thereafter for all things–including Marvel Unlimited.
For those elements that we don’t have original film or stats for, they are painstakingly recreated sourcing from the original books, to create a new black plate that can thereafter be colored in the same manner. Reconstruction technology of this sort has become so much better in just the last few years that this is a relatively simple process at this point.