We're pretty tricky nowadays with our billions of colors, but Dutch masters weren't exactly chopped liver. As spotted by blogger Erik Kwakkel, a certain "A. Boogert" even created a volume with individually painted watercolor swatches showing nearly every conceivable hue. Housed at the Bibliothèque Méjanes in Aix-en-Provence, France, the sheer level of detail in its 800-plus pages is on par with modern Pantone guides. We can imagine how useful that would've been to fledgeling Rembrandts, though we doubt very many actually saw it. Since printing color was a big challenge back then, only a single copy exists, and it likely cost more than a painter's wage to crack it. For a closer look, check the full version at the E-Corpus digital library.
Update: Blogger Erik Kwakkel has pointed out that it was him that first espied the ancient Dutch swatch book. The post has been updated to reflect that.