I am a Christian living in New Zealand and am supposed to attend a Muslim funeral. Could you please tell me what color is worn by mourners at a traditional Islamic funeral ?
No specific ruling has been given by Islam on this issue. Such matters have generally been left to the cultural traditions and customs of a society. These traditions among Muslims may not be universal. They may be different in New Zealand and in Britain for example.
So I would suggest that you follow the tradition of your area. What at best can be said is that the occasion demands a sober dress. Sports clothes, for example, would seem out of place.