Just one of a plethora of useful and long-established standards which privatisation has destroyed. Some TOCs' feeble efforts at first class denotation do make me suspect that they are deliberately trying to trap people into penalty fare situations! SWT is poor, Thameslink has been pathetic for years now and the new GWR livery (which I really like) has a near-useless pencil-thin silver stripe for first class. Only very recently has Southern apparently bowed to pressure and adorned its 5 car 377s with yellow stripes (albeit inconsistently), having run them for years with virtually nothing to identify the location of first from the outside.
Southeastern's new livery is the clearest by far, and I would like to see the universal adoption of very clear external marking, as it not only helps those of us who use first class, but also helps those who don't to avoid it. Of course, some of the more bizarre liveries these days do not lend themselves to proper identification but that's easily changed.
The other factor of course is that so many people seem not to care where they sit - either deliberately chancing it (and almost always getting away with it) or simply being so unaware of their surroundings that they haven't a clue (or care) that they are in first. This morning, my Southern 377/6 unit from Redhill gradually filled with people in first, but when the conductor strolled through (without checking tickets or saying anything) about half of them moved to standard!