can i just keep restarting the journey until i get it or do i need to go back to the menu and load it up every time?
Like others said restarting is how to do it. This way is best because the only way to make it selectable from menu would be to create a separate model with just that livery which would make it so that multiple trainbows show up in timetable even though real life there is only one (like what happens with TSW2 wrapped ACS64 in Boston).