If you’re talking about the 8 car trains then those are actually two 4 car units. The livery will only apply to the actual unit you’re driving, if it applies to the one you’re coupled to then it’s completely random, as it would with AI trains as the game considers it to be a seperate unit
I don't have a loading problem, but the rainbow livery for Class 395 didn't appear in the selection menu!
The "other" livery for the 375 is actually the 8 car unit, with the same livery, that is how you select between a 4 car and an 8 car set. The Trainbow livery is not user selectable, it randomly appears on the 395 that has the same number as the one IRL. It is just luck if you get it.
The trainbow cannot be selected in the menu , it is assigned to train 395012 so if the service that you choose is driven by 395012 then you will get the trainbow