Prussian blue was a mistake

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To create the color called carmine, one can boil the powdered bodies of cochineal — Dactylopius coccus — in an ammonia or sodium carbonate solution.

For his version of carmine, the paint maker Johann Jacob Diesbach (1670 – 1748) used a formula which called for potash and ferric sulfate. One time, when he set out to make a new batch, he borrowed some potash from his colleague. But unbeknownst to Diesbach, his colleague’s potash was tainted with animal blood. The blood, the potash, and the ferric sulfate reacted, and the result was that the red color Diesbach was aiming for instead turned out a brilliant blue: he had by accident invented what we nowadays call Prussian blue.

There has been synthetic colors in existence during earlier time periods. For example there was Egyptian blue, which is considered to be the first synthetic pigment. Egyptian blue was known for thousands of years but fell out of use some time after the Roman era, and consequently the knowledge of how to make it was also lost. This is why Prussian blue is called the first modern synthetic color.

Here’s a thing you can do with Prussian blue: Fern cyanotypes

And here’s another thing: Hooker’s green