Is he who considers that which exits to be semen, based on specific characteristics, without verifying the color and odor, mistaken; and does that affect his fasting and prayer in the event that he considers it semen?

It is clear that the questioner is afflicted with waswasah (whispers/obsessive thoughts), and it is advised to avoid and disregard these whispers. This question has been answered previously, stating that the fast is not invalidated, and there is no need for excessive questioning about this matter, as its origin is waswasah.