If you pickle that red onion, everything turns red. for example:

But that's not the reason... from chowhound

Red onions got their name because, at the time of their discovery, purple didn't exist as a color descriptor. Over time, a more complex language for colors evolved, and hues like yellow, green, and, yes, purple, entered our vocabulary. As with many quirks of language, names don't always evolve with our understanding of the world. (For example, the pineapple was named by a British explorer bereft of any understanding of the fruit.)

It may seem strange to think that specific names for colors are a matter of invention. After all, red and purple being two distinct colors seems like a simple fact today. However, it is important to note that language plays a big role in creating these variations. Pink, in particular, was not used as a color descriptor in English until the 17th century. And before the word orange was invented, orange items were often called red or geoluhread in English-speaking countries, which translates to yellow-red.