The color red is a primary hue found at the end of the spectrum. You might turn red with embarrassment. It also represents financial loss, as in the term "red ink."
The word red goes back to before 900, and we can see its roots in the Old English rēad. The idea of red as a signal to stop came in 1849. The color has been associated with rebellion as far back as the 13th Century, and in 1917, the Russian Bolsheviks adopted the color as their symbol. When the movement became the political party of Communism, the members became known as "Reds."