Oliver Byrne - The First Six Books of the Elements of Euclid (1847)
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Another one I didn't discover - Taschen reissued it a few years ago and apparently it had a cult reputation before that. But now you can flip through it (conceptually speaking of course) without having to buy a book that honestly we won't really read.
Byrne was a civil engineer and prolific author who developed a method of teaching Euclidean geometry using various colors. Or maybe just coloring regular illustrations - honestly I haven't read the details but from some mathematician comments it does seem to be more a solid idea than an eccentric one. What is definitely eccentric is to find a book in the middle of the 19th century using a long "S" which probably requires more an adjustment for readers than the coloring.
Not much is known about Byrne though his other works include a lot of what sound like routine engineering and math texts along with How to Measure the Earth with the Assistance of Railroads and Description and Use of the Byrnegraph, an instrument for multiplying, dividing and comparing lines, angles, surfaces and solids. I can't help but think in some alternate universe he did a colored text of non-Euclidean geometry and thereby invoked Cthulhu.