The beer style known for its dark color from roasted barley is stout.
Stout is a dark, top-fermented beer style that developed in Britain and Ireland. Its color and characteristic flavors come largely from roasted barley and other highly kilned malts, which can contribute notes of coffee, cocoa, toast, or dark caramel. The degree of roast varies, so not every stout tastes intensely bitter or burnt.
The name originally described a strong version of another beer, especially a strong porter. Over time, stout became its own recognized family, including dry Irish stout, milk stout, oatmeal stout, foreign export stout, and imperial stout. Guinness is the best-known example of dry Irish stout, though the style is made worldwide.
Porter is the closest common mix-up. Both styles are dark and historically related, but modern stout is especially associated with roasted barley, while porter more often emphasizes roasted or dark malt. Color alone is therefore not the complete definition.