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GordonWay2
Since: Jun, 2012
Daefaroth
Since: Jan, 2014
2016-05-28 23:59:57
https://www.youtube.com/watch?v=EwXK7gye9JI
is from episode 15 of Long Ago and Far Away and seems to fit.
Weirdwombat
Since: May, 2012
2016-06-04 05:18:43
Sounds like The BBC adaptation of The Crystal cave https://en.m.wikipedia.org/wiki/The_Crystal_Cave

A cartoon I saw long ago, featuring a Merlin-like character (who may have actually been Merlin) telling his young ward a story about a young boy who witnesses a battle between two dragons - one "with a belly as white as milk" and the other "red as wine". I believe the white dragon (who is described as such despite being gray with a white underbelly) emerges victorious. At the end of the story the old wizard pulls an "and that little boy was me", though not in so many words, only showing his ward an artifact from the event.
Edited by trulymadmoves