The Blue Rider. Why was he blue? Could he have been green or purple? Not really.
"Der Blaue Reiter" (German - The Blue Rider) was a group of Munich artists. It was formed in 1911. The two most important members of the group were Kandinsky and Franz Marc. And how was the name invented? Well, they both liked blue, Marc liked horses and Kandinsky liked riders... This very important group of modern art was later joined by Paul Klee.
(The picture titled The Tower of Blue Horses is from https://www.franzmarc.org)
Find out more about Kandinsky and Marc.