"Brown is the colour of his hair."
Translation:Brown yw lliw ei wallt o.
Things like "pobol y Cwm" (the people of the valley) or "pen y bryn" (the top of the hill) apply, I believe, also to things such as "lliw ei wallt (e)" - "the colour of his hair".
You can't add an extra "y" or 'r in front because it's already definite. That would be a bit like "the his hair colour" in English -- "his" makes it definite so you can't add another "the" in front of that.