Are the very scant yellowish discharges that appear as white moisture on the tissue considered yellowness, or must the yellowness appear in a clear quantity to be considered as such?

What you see as white on the tissue is not a yellowish discharge (ṣufrah), but rather normal vaginal moisture. As for the yellowish discharge, it is considered menstruation (ḥayḍ) if it is connected to blood at the end of the period and the duration is suitable for menstruation. If the yellowish discharge continues throughout the month, then the woman experiencing it is considered to have prolonged vaginal bleeding (istiḥāḍah). She should refer to her previous habit, or act according to sound discrimination, counting the period of blood as menstruation and anything else as istiḥāḍah.