Originally the stripes were Yellow first class, light blue second class, red for restaurant or buffet
Very few vehicles had the second class stripes, because second class was rare by time the stripes were introduced. And obviously it stopped when third class was reclassified as second.
The stripes have been revived for luggage / cycle spaces, and helps especially as GWR labels its coaches in fixed orders and regularly runs sets in any order, eg this one yesterday evening, a 165 formation lettered E D A B C
But using what was built as an inner suburban set on a 3.5+ hour journey is more a reflection on GWR relying on any thing diesel powered that moves, because the 175s are so unreliable