Red Onions (and every other not-red food that’s called red) is older in the English language than the word “purple”.

Purple is a relatively modern concept in English having first been used circa 900AD. Before that basically everything towards the magenta part of the spectrum was all just called red.

See also Orange, the colour is named after the fruit and not the other way round.

See also: ‘robin red breast’ to describe the European robin, which very clearly has an orange breast: