Is the light, clear discharge on the sixth day of the menstrual cycle, after wiping with a tissue and nothing appearing, considered purity, or must one wait for the "white discharge" (al-qassah al-bayda') to be certain?

What you saw on the sixth day of dryness is purity (Tuhr), and the sign of purity is the cotton swab coming out clean without any trace of blood, yellowish discharge (sufra), or brownish discharge (kudra). Normal white discharge does not cause any harm. You must perform ritual bathing (ghusl) immediately upon seeing this sign of purity, and do not wait to see the white discharge (al-qassah al-bayda'); because any sign of purity that appears first is considered a valid purity.