The two saints, Cybi and Seiriol, lived on two islands off the coast of Anglesey, Holy Island and Puffin Island. They were friends and they met each other at the well of Clorach, about halfway between the two islands.
The sun would shine on Cybi's face as he walked towards the east in the morning to meet his friend, and then again as he walked westwards at night.
On the other hand, Seiriol's back was to the sun in the morning and also at night. Consequently, Cybi had a tan and his face was yellow but Seiriol's face remained pale. That’s why they were called Seiriol Wyn and Cybi Felyn.