Colour-blindness was discovered after a Swedish railway accident in 1875 where two express trains collided head-on after one of them ignored a red stop light.

The existence of colour-blindness had already been proposed theoretically by a philologist named Lazarus Geiger to explain strange descriptions of colour in ancient texts, first pointed out by Gladstone (yes, the same Gladstone who would become British Prime Minister). Geiger believed that our modern sense of colour had only evolved fairly recently.

In the investigation of the railway accident, an ophthalmologist named Frithiof Holmgren thought that it might have been caused by one of the train's crew having mistaken the red stop signal for a white go signal. Both the driver and the engineer had been killed in the crash, so he couldn't test them- but he did test other employees of the same railway company and found that several of them were colour-blind.

At the time, colour-blindness was thought to be a genetic reversion to a condition that was normal in ancient times. In 1877, Hugo Magnus wrote a book on the subject which suggested that people would eventually evolve to be able to see ultraviolet.

(Source: Guy Deutscher, Through the Language Glass)