According to Edible Geography, “one of the earliest appearances of 'chocolate'-colour, in Abraham Werner’s 1821 Nomenclature of Colours, is as a modifier for a particular shade of red. 'Chocolate Red,' Werner writes, is 'a veinous blood red mixed with a little brownish red.'”
“Chocolate” was a deep red because that's the color of unprocessed cocoa beans. Chocolate didn't take on its now-characteristic brown color until chemists got their hands on it, says Edible Geography.
“In 1828,” says Smithsonian magazine, “a Dutch chemist found a way to make powdered chocolate by removing about half the natural fat (cacao butter) from chocolate liquor, pulverizing what remained and treating the mixture with alkaline salts to cut the bitter taste. His product became known as "Dutch cocoa," and it soon led to the creation of solid chocolate.”
Where natural cocoa is a deep red, Dutch chocolate is a darker brown. After its invention, unprocessed cocoa, with all its bitterness, fell out of favor. This new sweet chocolate, “cheap and brown, and its colour association shifted to match,” says Edible Geography, came to erase older ideas of cholocate—even though it's still possible to buy chocolate in an array of subtle shades.
Here, for instance, are the five varieties that King Arthur Flour sells, along with natural cocoa—labeled "F"—for comparison:
Natural cocoa also gave the cake a reddish color – which is exactly why chocolate cakes were often called devil’s food cakes (red devil, get it?), back when natural was the only cocoa available to most bakers.
source: smithsonian.com By Colin Schultz