"A white wedding dress."
Translation:Ffrog briodas wen.
Why is gwyn not acceptable? We use it to describe other things, e.g. gwin gwyn.
Gwen is the feminine version of gwyn, so as ffrog is feminine, "a white dress" comes out as ffrong wen. However, not all speakers use feminine nouns all the time, so I guess ffrog briodas wyn (with the mutation) should be accepted.
It is easy to forget about masculine and feminine in Welsh as it isn't always very obvious. A good tip for this sentence is that clothes are almost always feminine, except for cap "cap", crys "shirt", trowsus "trousers" and usually tei "tie", which are masculine. (In your head picture a guy wearing a cap, shirt, tie and trousers!)