The last colour to get a name is almost always blue
In the 1960s two researchers worked through the colour words of around a hundred languages and found they arrive in a fixed order. A language with only two colour words has BLACK and WHITE. The third is always RED. Then come GREEN and YELLOW, and only after those does anyone bother inventing a word for BLUE, with BROWN, PURPLE, PINK and GREY turning up later still.
Ancient Greek seems to have been stuck at an earlier stage. Homer calls the sea wine-dark, honey green and sheep violet, and never once reaches for a word meaning blue — which William Gladstone noticed in the 1850s, counting colour words in the Odyssey some years before he became prime minister.
Orange was a fruit, and magenta was a battle
English had no name for ORANGE until the fruit arrived. The colour was described as yellow-red, and anything in that range was simply called red instead. A robin still has a red breast, a fox is still a red fox, and people with unmistakably orange hair are still redheads.
The fruit travelled from southern Asia through Persian, Arabic and Spanish before reaching English in the 1300s. The colour name followed about two hundred years later, borrowed straight from the thing itself.
Several shades in this grid are pointing at something else. TURQUOISE means Turkish, because the blue-green stone reached Europe by way of Turkey. INDIGO means Indian, after the plant the dye was pressed from. MAROON is the French word for a chestnut, and VIOLET is just the flower.
MAGENTA has the oddest story of the lot. The dye was invented in 1859, the same year a famously bloody battle was fought near the town of Magenta in northern Italy, and its makers named their new colour after the headlines. GOLD, SILVER and CREAM are doing the same job less romantically: naming a shade by pointing at an object and trusting that everyone has seen one.
Pink used to be the boys' colour
An American trade magazine for children's clothing shops told its readers in 1918 that the accepted rule was PINK for boys and BLUE for girls — pink being the stronger, more decided colour, and blue the more delicate and dainty one. Pink was understood as a small red, and red was a soldier's colour.
The modern arrangement only settled in the middle of the twentieth century, and it settled because it sold more clothes, not because anybody discovered anything. Handy to have in your pocket the next time a four-year-old announces that a colour belongs to somebody.
Why a rainbow has seven colours
It does not, really. A RAINBOW is a smooth smear with no edges in it anywhere, and the place where one shade stops being called RED and starts being called ORANGE is a decision made by people rather than by the sky.
Seven is Isaac Newton's doing. He could comfortably have named five, but he wanted the spectrum to match the seven notes of a musical scale, so he added orange and INDIGO to the list. That is why children still learn a colour almost nobody can point to — ask anyone to find indigo sitting between BLUE and VIOLET in a real rainbow and watch the confidence drain away.