When the great Swedish chemist Karl Scheele first began looking for ways to take the rich green tint of copper and apply it into a pigment form for dyeing and painting, his main requirement was that it would be durable. The copper carbonate compounds of the late 18th century simply faded away in fairly short order, rendering many beautiful paintings worthless, and making green an unsuitable choice for design and fashion.
His solution, arrived at in 1775, was another salt altogether, famously called Scheele's green, which proved to be not only a durable, but an altogether deep and beautiful green. Like Prussian blue and cadmium red, it became one of the centerpieces of Victorian-era Europe's color schemes, adorning dresses, artwork, playbills - even as a dye for candies and other processed foods of the rising Industrial Revolution.
As it gained in popularity, several scientists noted that when the dye became damp - especially in large quantities, such as entire rooms decked in Scheele's green wallpaper - the smell it gave off was a hint of garlic. This was no surprise to Scheele, whose major work (including Scheele's green pigment) was derived around the element which caused the smell: arsenic.
Anecdotes of children frittering away to slow respiratory deaths in brightly-colored green rooms, newspaper printers collapsing in back rooms overcome by "vapors", and entire troupes of women bedecked in lush green dresses succumbing to the effects of arsine gas litter chemist magazines of the 19th century. In 1980, one scientist put forth the theory that Napoleon, already suffering from a fatal stomach ulcer, was hastened to his deathbed by Scheele's green wallpaper that adorned his exiled villa on St. Helena.
Eventually, Scheele's green was exceeded by other (equally deadly) greens of the era, including Emerald green and Schweinfurt green - and, ironically, these greens met their demise because copper - not arsenic - fell out of favor, replaced by zinc and cobalt.