Is it "preservation" if it's a livery never carried by that class of loco?
It's the locomotive that's preserved. It'll need painting periodically anyway, so why not have a bit of fun with it?
It might not be "authentic" but they are working machines as well as museum pieces. For locos on static display which are unlikely to run again, I'd stick to the 'canon'... but the running examples already break canon with the coaching stock they work with (a couple of years ago I rode in a train of Gresley Teaks behind a Western for example!) so while it might annoy photographers it doesn't strike me as a major taboo. As long as there are still some authentic examples around, that is. I'm not sure I'd be keen on 89001 getting Virgin colours or one of the two surviving Warships having FirstGroup's "dynamic lines" applied to it.
I quite like some of the "what if" liveries we've seen over the years, the Hymek that wore Dutch for a period looked great in my opinion. I'm less taken with the LoadHaul examples, as I feel that's a massively overrated livery... but that could just be my inner contrarian reacting to all the "foaming" about it!