Green

In past centuries, artists had all kinds of difficulties with green pigments. First, none of them were permanent, second, most were either weak or toxic. Of course, greens could, and always can be made by mixing blue and yellow, but there were also other sources in nature:

In ancient China, a greenish earth was used to make porcelain, but it was very pale in color. Malachite was ground to obtain a beautiful tone of green, but it could not be ground finely, or it would loose its color. Suspending copper over a bath of vinegar would produce a patina of another green, however it was corrosive and short lived.

In the late XVIIIth century, Scheele, a swedish chemist, experimented with arsenic, when he accidentally produced a wonderful, but highly poisonous green. He knew about the toxicity, but was advised not to disclose it, as not to jeopardize the potential economic success. It became known as Scheele-Green and was indeed immediately very well received by society. Textiles, carpets and wall papers in green became the latest must have in fashion.
By the way, Napoleon, who was exiled in Santa Helena and lived in a room decorated with Scheele-Green wallpaper, died in 1821. It was later determined, that his body was preserved like a mommy because he died of arsenic poisoning.