Pity the poor rubricator - the medieval chap whose job was to write in red ink. He only got to do the titles and initials of chapters and the important dates on calendars – the red letter days. It was one up from being a scribe (black only) but was he was jealous of the illuminator who was more skilled got to use blue and gold?
Why red we wondered? Basically because it was cheap and plentiful. If you wanted blue you’d have to buy Lapis Lazuli and make ultra-marine. Given that in those days Lapis Lazuli came from north east Afghanistan via camel trail then ship this made it incredibly valuable.
Rewind to Roman times and they circled the important days on their calendars in red. If you go back even further to the Egyptians and look at what the average Hieroglyphics pencil case looked like there were only two colours. Guess what?
Further back still to cave drawings and yup you've got it …