#191 looks weathered because it has sat out in the weather for four years now. The museum has neither the resources or staff to repaint every locomotive every couple of years just to keep them shiny.
Also, #191 was painted with paint of different textures and slightly different shades of black, in order to emphasize various different parts of the locomotive. I can't remember without looking at the locomotive again up close, but the cab may have been painted with a "flat" black paint that may make it look more weathered than it really is. I helped paint it, and we used alkalyd-modified gloss acrylic paint, one step down from Inrom. It should look good for several years more.
Mike