Is the yellow fluid or saliva upon waking considered impure?

Human saliva is pure in its origin, and its yellowness generally does not render it impure, as it may be due to food residue in the mouth. Therefore, as long as it is not mixed with blood or it is not certain that it originated from the stomach, it is pure. However, if it is certain that it is mixed with blood or originated from the stomach, then it is impure.