Why "wen" and not "gwyn" for white in this sentence?
cath is a feminine noun, so the adjective describing it, gwyn, has to be mutated -> wyn. Secondly, gwyn has a feminine form which is often used, gwen -> wen:
Thanks for that! Is there a section where male/female/mutated adjectives is covered?
Not that I know of, however if you do the Spanish course it will be covered there, which might give you a slightly better understanding of it in Welsh