Romulan Type 2 (Brown) was a material option, an Alpha Quadrant Alliance designation for the visual appearance of a spacecraft's outer hull, i.e. a vessel's "skin". Romulan Type 2 (Brown) was available for some warbird classes of the Romulan Republic and its Romulan Republican Force. Type 2 (Brown) stood out from other Romulan hull materials because it put the ship in a uniform, brown cover, rather than the usual shades of green. The warp plasma in the nacelles appeared bright green.
In 2379, the pair of Mogai-class warbirds participating in the Battle of the Bassen Rift, were differentiated by their hull material. While the IRW Valdore was green, Commander Suran's IRW Soterus was brown. (TNG movie: Star Trek Nemesis)
Related ship classes are sorted together, rather than alphabetically.