From the accompanying article:

Mary Gartside’s An Essay on a New Theory of Colours is the expanded edition of An Essay on Light and Shade (1805), “one of the rarest and most unusual books about colour ever published”, claims Alexandra Loske, a historian of colour and curator at Brighton’s Royal Pavilion. On the surface, this tract appears to sit neatly within the tradition of instructional artist’s manuals. Indeed, Mary Gartside worked as a watercolour teacher and botanical painter, exhibiting her drawings at the Royal Academy in 1781. And yet, the Essay makes use of an intellectual palette whose spectrum exceeds Gartside’s pedagogical contemporaries. It is best remembered, rather, as an exemplar of the myriad early-nineteenth century treatises on colour — works inspired, in part, by the newfound availability of novel pigments.

This sounds really interesting, but what is it exactly that is so notable about her "intellectual palette"?

Here's a quotation from the Wikipedia article:

She published three books between 1805 and 1808. In chronological and intellectual terms Mary Gartside can be regarded an exemplary link between Moses Harris, who published his short but important Natural System of Colours around 1766, and Johann Wolfgang von Goethe’s highly influential theory Zur Farbenlehre, first published in 1810.[1] Gartside's colour theory was published privately under the disguise of a traditional water colouring manual. She is the first recorded woman known to have published a theory of colour.

Goethe's theory of colours was one of the most influential because it made what was then a novel proposal, to the effect that colour was not seen directly but through a cloudy medium, through which the prismatic colours were projected in some way onto the eye. He also set up a "colour-wheel" on the basis of psychological observations of the effects that colours have on people, esp. in juxtaposition. This is something that Gartside to some extent anticipated, as she spends a goof half of the treatise discussing how painters should arrange colours—both theoretically, as prismatic colours [here G begins from the middle of the spectrum, but knows full well that the two ends meet], and in practice, so as to achieve maximum clarity and brilliance.

Sorry, I don't buy that sort of vague wordy colorthink. https://www.handprint.com/HP/WCL/book3.html#goethe

The punchline:

In sum, this book about color by a man who was neither an artist nor a scientist offers neither practical artistic guidance nor a valid scientific approach to color. It offers instead a fascinating case study of error and falsehood. Oblivious to the grossly censored and misinterpreted transmission of Goethe's true gospel, many authors today still treat him as the quotable old testament of "color theory" in what has become a ritual display of color erudition. As I've taken pains to show, ritual is never erudition enough.