The purpose of this sentence is to practice an emphatic pattern, here to emphasise that his hair is brown, not black, green, fair, etc. In Welsh this is very easy to do by moving the element to be emphasised to the beginning of the sentence but yes, the English equivalent sounds a bit unusual as English generally relies on a tone of voice alone or on using a different pattern altogether.
The unemphatic pattern in Welsh would be:
- Mae ei wallt yn frown - His hair is brown.
Yes but the correct English is still "His hair is brown" or "He has brown hair". Mangling English does not help us to learn Welsh. Maybe get a sentence which actually says, "He has brown hair, not grey,"? Then, if the answer was not "Brown yw lliw...." you could mark it wrong!