Which London Underground line is coloured dark blue on the Tube map?

The Piccadilly line is coloured dark blue on the London Underground map.

The line runs between Cockfosters in north London and Heathrow Airport or Uxbridge in west London. Its dark-blue map colour is one of the long-established visual conventions used to distinguish Underground routes.

The Piccadilly line opened in stages in the early 20th century. Its central section originally formed part of the Great Northern, Piccadilly and Brompton Railway, which began service in 1906. The route was later extended to serve areas including north London and Heathrow.

Passengers sometimes confuse the Piccadilly line’s dark blue with the Victoria line’s lighter blue or the Northern line’s black. Map colours are not simply decorative: they allow travellers to follow routes, identify interchanges and understand the network quickly, even when several lines share a station.