I remember when I was growing up, purple onions were always mild, and things that were often used raw, in salads, and similar things. However, unless you know the variety you are getting, now there can be very potent purple ones out there! I remember I noticed this many years ago, when I could hardly get through just peeling the first of a batch I got - the first time this ever happened with a purple! I only occasionally buy them, usually a price thing, because they were the cheapest, which seldom happens, but a while back yellow onions got more expensive than others, for some reason.
Fortunately, this season the price came down.