Don't wear anything United related, don't in anyway let anyone know you're a United fan and don't in any audio or visual manner show support for United, mainly for your own safety, (there are still a number of thugs who attend football matches who may 'take offence') although this is becoming less of a factor these days, (thank fuck), but as has been said you'll be escorted out of the ground 'for your own safety'.

Also, it may be worth coming up with a reasonable 'cover story' as to why you support Hull. No disrespect to Hull but they don't have many international fans, so as soon as anyone hears you speak you're gonna get asked why you're at a Hull match. So you could just be a fan of EPL football in general and Hull was all you could get (be careful with that), or your great granddad was from Hull before he emigrated (where in Hull was he born?), or you married a local girl (where in Hull did she come from?). Or you could just try and keep your mouth shut.

This isn't the 70s though, so your not likely to get in trouble, just that it can get somewhat nervous, especially when United score.