Why every astrology app is purple

Almost any astrology app uses the same four colours. A background of indigo so dark it is nearly black. A violet gradient washed over it. Small white dots standing in for stars. Gold or cream lettering with small strokes on the letter ends, called serifs.

None of that shows anything the app worked out. Under the colour is a table of numbers, and numbers have no colour. The look is made of three borrowed promises, and each one can be dated.

The first is money. For thousands of years the good purple dye came from sea snails, and the yield was tiny: about twelve thousand snails for 1.4 grams, enough for the trim of one garment. A Greek writer of the fourth century BCE said purple sold for its weight in silver. Then in 1856 an eighteen-year-old student, William Perkin, made purple by accident in a laboratory while trying to make a malaria medicine, and purple became cheap. The feeling that it means wealth stayed on anyway.

The second is the spiritual cosmos, roughly a hundred years old. In 1927 Charles Leadbeater laid the seven colours of the rainbow onto the seven chakras, violet at the top of the head. A researcher who traced that system found the rainbow colours and the modern list of qualities first appeared together only in 1977. Indian astrology hands out colours of its own through the nine planet-gems, and no purple is among them.

The third is the night sky, which does not supply it either. The eye sees almost no colour in the dark, because the parts that handle colour stop working in dim light. The famous purple space photographs are coloured on purpose by astronomers, so different gases can be told apart.

So the look is a mood, and a mood is not a result. Notice where it stops. In most of these apps the account page, the price list and the terms are plain type on a plain background. The half meant to be felt gets the colour. The half where a number has to be exactly right does not.