My first suspicious would be a new set of dyes. The first synthetic dye (mauveline, which is even a bit violet!) was discovered in 1856, and sparked a revolutio

My first suspicious would be a new set of dyes. The first synthetic dye (mauveline, which is even a bit violet!) was discovered in 1856, and sparked a revolution in colors. I found this paper, which analyses a Monet sea painting (https://www.artgallery.nsw.gov.au/collection/works/8356/ https://www.artgallery.nsw.gov.au/collection/works/8356/) and says > Nine different pigments were found on the painting. Many of the identified colors were modern pigments that became available only late in the 19th century as a result of scientific advances in pigment chemistry. Although similar colors were available in a natural mineral form, they lacked the vivid color of their manufactured counterparts. The use of these new synthetic metallic oxide colors by Monet accounts for the brilliance of his paintings. (https://www.cambridge.org/core/journals/microscopy-and-microanalysis/article/monets-painting-under-the-microscope/F66592EA744E3B6DB1F94DCAE52742E9 https://www.cambridge.org/core/journals/microscopy-and-micro...) The nine pigments are lead white, vermilion, cadmium yellow, chrome yellow, lead chromate, viridian, emerald green [copper acetoarsenite], cobalt blue, and French ultramarine. I guess of these, lead white, vermilion, and french ultramarine have been used historically, but the other six were first synthesized in the 19th century. One could imagine that having access to cheap bright colors influenced art...