According to reports and research, it was discovered that Then red was used as a primary descriptive colour that was used to explain or name anything that wasn't black or white in colour but years after when we started to form new and more words that we discovered new words to describe other colours.
According to Allrecipes.com Red onions have also earned their "red" name because their purplish skins have historically been used to make reddish dyes.
What you know as purple onions can sometimes be red because they depend on the quality of the soil and also the growing conditions for their ultimate appearance. You should know that A soil's pH balance can significantly change how red an onion appears.
These are the results found out online after researching on why they are called red onion instead of purple.