From Oxford’s Bodleian Library we get a wonderfully fancy initial, with monsters and flowers, two of my favorite things. I think this one would work particularly well as a coloring page. The whole coloring book is here.
If you want something more modern, how about this great abstract design from the Smithsonian Libraries? It doesn’t have as much detail, but it could be very bold, with lots of scope for experimenting with color choices. The whole coloring book is here.
The New York Public Library gives us this image of an astrolabe, which could make for a particularly interesting coloring page because in the original the lines aren’t intended to outline shapes or build up an image, yet they divide the space into lots of interesting areas with lots of possibilities. What would it look like with the emphasis on shapes instead of lines? The whole coloring book is here.
[Pictures: Mechanical Hand, wood block print from The Works of that famous chirurgion Ambrose Parey, translated out of Latine and compared with the French, 1634;
Initial S, wood block print from Lucain, Suetone, et Salluste, 1490;
Largo, woodcut by Oswald Herzog from Plastik: Sinfonie des Lebens, 1921;
Astrolabium Physicum, engraving by Martin Waldseemüller, 1517.]