Is there any reason against using red onion, or a mixture of red and yellow onions to make an onion soup? 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.