Quite true, but at least you have a little more notice if the outside of the train is clearly marked. The fragmentation of the network into various TOCs has seen a perfectly good livery standard disappear. When I catch my morning train (booked as a 377/6) I try to place myself such that I can get to the first class at the front or rear as soon as I see the yellow stripe's position as the train approaches.
I'm very much of the opinion that only First Class ticket holders are actually
regularly able to understand what the yellow stripe means, particularly on commuter/regional TOCs like Southern and Southeastern.
I would never have known before I had interest in, or employment on, the railways, which was when I was a predominantly Standard Class passenger, and I'm sure that even if you pointed to it and said "what does that mean?", most passengers would just give you a funny look and shrug.
Given how many people simply don't notice the stickers on the windows, white antimacassars, green and white stripes on the doors, massive "First Class" lettering on the outside, penalty fares notices etc. - aside from your regular faredodgers and chancers - it makes you wonder if it's ever noticed much at all nowadays.