can I use red onions in making onion soup...that will be as tasty as using regular ones?

You can use yellow, white or red.

When I am going to make onion soup I get the onion that are on sale. Once they have been cooked down and are all caramelized you do not know if the onion was red, white or yellow.

They will all taste just as sweet.

Once you have caramelized them and simmer them in the stock the colour of the onions fades to nothing or brown, I to use several types including red, yellow cooking onions, sweet Spanish and Vidallia onions, some white of leeks and shallots, all of them imparting a different flavour, and texture, but over ll undetectable once cooked in the soup as for the type.

No, not a good idea, they don't have the same intense flavor as yellow or white onions, the soup will be bland and tasteless. Red onions are really only good thinly sliced and used raw.

Of course you can. When I make, I actually use a blend of yellow, and red onions.

It will be a little sweeter though.

You want sweeter onions for caramelizing.

Of course you can.