Don't think metallic black with gold doors would work well. Easy to imagine a night-time scene at a station with yellow glow lamps and black sky making the train a little hard to notice for some.
Liveries seem to work best with a dark band across the window level on carriages, with lighter colours elsewhere, such as the old Intercity Livery, but doors should indeed be a different colour like the old Central Trains colours.
Isn't the term "gammon" racist? There's certainly no way a none white person with the same views could be called that. People are too fast to label anyone with differing views under a tagline, in a sort of monster-fication. The terms "homophobe" are thrown at people who don't have any hatred, but think that marriage is traditionally a joining of a woman and a man, "transphobe" is thrown at anyone who is concerned that women's rights might be at risk, "woke" on the other side is used on anyone who is offended by something commonplace, and there are many more examples, we need to see others as ordinary people, not as opposing groups.
Labels make people seem unreasonable and alien, when the majority of people are the same, they all buy an overpriced ticket, avoid other passengers if they can, moan about the delays, and get off at their destination. It's only on the internet where people have free reign to shout that discussions and these arguments take place, in the real world we all act virtually the same, politics, sexuality, religion, are all rightly private, even if they do influence us.
Just imagine if ahead of the "Brexit" vote, the government painted a train in EU colours...
Would passengers travel on it like before? Yes.
Would some be smug and others annoyed? Yes.
Would it change anyone's opinion on the subject? No.