> Michel Pastoureau’s book on blue begins by highlighting the neglect this colour faced among the ancient Greeks and Romans, who rarely wrote about it or used it. He even explores the intriguing question of whether ancient peoples could perceive blue at all!

The first synthetic pigment was calcium copper silicate or Egyptian Blue [1], so called because the Egyptians manufactured it from at least the fourth millennium BC; from the Egyptians, the rest of the Mediterranean learned to make and use this artificial pigment, so that it is widely attested in art from the Minoans, Mycenaeans, Greeks (if distinguishable from the Mycenaeans), Romans, and so forth up until the middle ages. Given that Egyptian Blue is a synthetic pigment that must be manufactured by human skill and ingenuity, it boggles the mind that people keep falling for this idea that ancient peoples could not perceive blue. I have no idea how someone could write a book suggesting that ancient people did not write about (Plato certainly did) or use a color that they in fact synthesized, manufactured, and used in art. The ancient Greek word for the color is κυανοῦς, the Latin caeruleus (but of the eyes, caesius).

[1] https://en.wikipedia.org/wiki/Egyptian_blue

One thing I find fascinating is how crass many medieval objects and garments are colored, often to the point they are offensive to our modern tastes. Our fiction depicts the Middle Ages as a mix of gray and earth tones, but reality is the opposite: people in the Middle Ages loved colors (just as the people before them). It's the wide availability of synthetic colors that lead to us using them less and less in modern times, preferring everything to be muted, gray or colored in black, off-white and earth tones.

> Medieval scholars inherited the idea from ancient times that there were seven primary colours: white, yellow, red, green, blue, purple, and black.

These correspond to the classic tinctures of heraldry: the "metals" or and argent (silver and gold, conventionally depicted as white and yellow), and the "colours" gules (red), vert (green), azure (blue), purpure (purple), and sable (black).

However, iirc, purpure as a colour in its own right was a later addition, and was originally just considered a variant of gules (mediaeval heraldry lacking Pantone numbers or rgb/cmyk specs -- as long as the chosen paint or fabric was recognisably green or yellow or whatever, it counted).

> Finally, yellow or gold knights were rare and blue knights nonexistent.

The nearest thing to a canonical version of the Arthurian romances in English is probably Sir Thomas Malory's Le Morte d'Arthur, which may be too late (c. 1470) to count as strictly mediaeval, but which does include a single example of a blue knight:

> So leave we Sir Gareth there with Sir Gringamore and his sisters, and turn we unto King Arthur, that at the next feast of Pentecost held his feast; and there came the Green Knight with fifty knights, and yielded them all unto King Arthur. And so there came the Red Knight his brother, and yielded him to King Arthur, and three score knights with him. Also there came the Blue Knight, brother to them, with an hundred knights, and yielded them unto King Arthur; and the Green Knight’s name was Pertolepe, and the Red Knight’s name was Perimones, and the Blue Knight’s name was Sir Persant of Inde[0].

-- Le Morte d'Arthur, book VII, chapter xxiii.

This same part of the Morte also features (briefly) a brown knight, a colour not discussed in the article.

[0] inde: dark blue; etymologically related to "indigo", both words being ultimately derived from the Latin indicum.

My favorite fun fact about the colors monk's habits is that The Order of Friars Minor Capuchin, a branch of the Franciscans are at the root of the words cappuccino and capuchin monkey.

In Hinduism, since vedic period there are numerous references to blue. Krishna (Vishnu) is typically depicted in blue and one of his adornments is peacock feather, with shades of blue and green. Shiva's neck is blue as he swallowed poison to save his devotees and so called 'Neelakatnan' (neel-blue kanta-throat). There are also other references to blue in secular literature in Tamil and Sanskrit.

I am not sure white, grey and black are colours at all, since they are just degrees of illumination. To me a colour has distinct series of wavelengths. Maybe I'm wrong.