Colors appear to be added by the restoration process. This kills originality of the works. I would prefer to see an artwork as it was created, not "enhanced" in anyway.
The original scans with faded colors and yellowed paper are also available: https://www.c82.net/naturalists-library/sources/scans
The original scans with faded colors and yellowed paper are also available: https://www.c82.net/naturalists-library/sources/scans