What is the ruling on someone who swallows their saliva that carries a yellow color from the remnants of food after waking up from sleep, and does that affect the validity of their fast?

Your fast is valid and there is no make-up day required of you, because the change in the color of saliva resulting from food remnants is difficult to guard against. It does not invalidate the fast even if you intentionally swallow the discolored saliva; this is the view of the Malikis. The Shafi'is hold that a change in the color of saliva does not invalidate the fast if no trace of the food remains in the mouth. However, if a trace of the food does remain, then swallowing the saliva at that point would invalidate the fast.