Seoul buses are color-coded by role. The long-distance 'trunk' bus that crosses the city center is mainly what color?

Seoul's bus system uses color coding to communicate a route's role at a glance: blue buses are the trunk routes running long distances along major roads through the city center, green buses are branch routes serving neighborhoods and connecting to subway stations, red buses run wide-area routes between the suburbs and downtown, and yellow buses circulate on short loop routes within a district. Blue therefore signals the backbone, high-capacity routes crossing central Seoul.

Yellow and black are not correct here — yellow denotes the short circular routes rather than long trunk lines, and black isn't used as a route-category color in Seoul's bus system at all.

Once a rider learns the four-color scheme, it becomes a genuinely useful shortcut for guessing a bus's general purpose before even checking a map, since the color alone hints at whether it's a long cross-city ride or a short neighborhood loop.