Well, old paintings from that period are usually fine too, right? Pigments were usually made from mineral sources that last very well. It's usually the varnish on top that changes color over time.

A book's illustrations don't have that problem, and the pages aren't exposed to the air or sun as much either, if kept on a shelf in a dry place most of the time.

Does make me wonder how expensive the blue pages were to make though:

https://en.wikipedia.org/wiki/Ultramarine