Should I use yellow onions, red onions or even a mixture of both to make the best onion soup?
I realise this is a subjective question, but is there any reason I shouldn't use red onions or is there an advantage to using a combination of onions?
I would simply use whatever onions I had available! Red onions have a milder flavour than yellow onions (therefore red is usually preferred in salads) and obviously different colour but the difference is still small enough to interchange with no problem. Unless, of course, you desire a particular colour in the soup or are particular about the soup having a strong or mild flavour. Leek would make it even milder and sweeter. If I'd have to make a choice I would use yellow onions and spare the red ones for raw/decorative use.
There really isn't a reason not to use both other than preference. I make a delicious 5 onion soup that uses both red and yellow along with a few others of course. I know you didn't ask but to ensure it doesn't just taste like onion water I caramelizs my onions for the soup as well.