Eric Kwakkel found this extensive 800-page book on how to prepare and mix watercolour paints in an online library in France.

It was published in Dutch in 1692 by one A. Boogert:

He explains how to mix the colours and how to change their tone by adding “one, two or three portions of water”. To illustrate his point he fills each facing page with various shades of the colour in question […]. To top it he made an index of all the colours he described, which in itself is a feast to look at.

The book is called ‘The Clear Bright Mirror of the Art of Painting’ (‘De Klaerlighte Spiegel der Verfkonst’) and is written in plain Dutch. Unfortunately I keep tripping over Mr Boogert’s handwriting, otherwise I might have treated you to a couple of paragraphs. Due to the nature of the work (three colour printing wasn’t available until the late 19th century), it is likely that the author produced only a single copy. And it’s very cool is that this copy survived.

Shown here are two opposite pages of the index (“blatwijser of regisster”).