I have seen rags get sussed in EL3 and get battered, I have seen a crowd of liverpool fans in North 1 and nothing was said (though as they were the teachers and parents of a group of school children, it would have been a raggish thing to do to them, especially as two of them live within about 4 houses of me).
If it is an ordinary supporter and they are minding themselves and watching the game, no real problems (though would obviously prefer a blue in the seat), if they are getting lairy then they will get what they deserve no matter what stand of level they are in.
Drank in many a home fans pub and it has been obvious to all that we were City fans, never any bother, but as we have been sitting having a few pints and a chat about all things football, not likely to be, if we were in the same pub and started singing Blue Moon, City till I die, or other City songs, things would have been different.