I'm not sure about the affects of yellow toward red, but is sounds logical since yellow is from a Non Red gene. IME red is quite dominant (if the yellow wasn't there) . . . 2 generations of no red what so ever (green dragon) suddenly produced green red in F3. So do not breed the ones that carry even the slightest red. Perhaps Darth's method of cleaning reds by using yellow would work.
I agree to breeding back to female F0 (original mother) - as far the line as possible. Then breed siblings with no red nor yellow to get clean solid color. I would not breed back to male F1 because theoretically he carries 50% of male F0 genes - which carries red.