The swirls and fancy designs tend to break up the massive size of the unit. This makes them look a bit more manageable and does not present a solid wall of color. Think of camouflage, not that the designs serve that purpose but unrelieved solid colors actually stand out more than these swirls. Many Prevosts are two tone, solid dark upper color over a lower lighter color. THey look even more massive.
Over the road buses are solid because it is much less expense to paint and repaint - ask me - we are "camping" in a paint bay tonight waiting for the clearcoat to set up and avoiding threatened rain. Tiffin uses 4 colors and very elaborate designs.