I think one of the things for a museum that may aim to a wider audience (or other family members who may visit with a person of keener interest) is to show exhibits that can strike a chord with a visitor - particularly when there is an element of social history about it. That can mean displaying things that are generally historically accurate, but also from different periods - so a family of different generations may have someone who was a child in the 50s and commuted to work in the 70s but whose daughter commuted in the 80s (say) and livery schemes they recall from those periods is a way to strike that chord - eg Green / Blue / Blue Grey / NSE etc in this case.
Being able to take a young child round and say 'nana went to work on a train like this every day for years and years' is a way to convey that history. So there can be an argument for mixing it up a bit.
Still, at the end of the day it's up to the people who put the sweat and cash into preserving things, and I guess the group will be thinking about all such issues now they have a nice dry place to keep this important assett!