From the Historic England entry:
“Leslie Green (1875-1908) was appointed Architect to the UERL in 1903 and designed 40 stations for the company in a distinctive Edwardian Baroque house style (a small number of stations, such as Regent’s Park, had no surface building). Stairs, corridors and platforms were faced in glazed tiles with directional signage, produced by various tile manufacturers, each station with its unique colour scheme.”
From: London’s Underground: The Story of the Tube (2019), by Oliver Green:
“Each station had a different colour and pattern of tiling on the platform walls to help regular travellers identify their stop. A myth emerged later that the system of different tile designs was intended to help passengers with reading difficulties, but there is no evidence for this. Literacy levels in Edwardian London were not particularly low, and two generations had been through the Board Schools established after the Education Act of 1870. By the 1900’s, nearly all Londoners could read and write.
‘Way Out’ signs and the station name were fired into the tile work, although the name only appeared at either end of the platform and could easily be missed from inside a train. A continuous enamel nameplate frieze, repeating the station name all along the platform wall, was introduced much later in the 1930s. The renovation of the original tiled station names leaves room for occasional confusion where the name has been changed and both the current and original versions now appear, for example at Hampstead on the Northern line (once Heath Street) and Arsenal on the Piccadilly (once Gillespie Road). The latter has a special claim to fame in being the only Tube station in London renamed after a football club. However, Arsenal is no longer the nearest station to the club’s new Emirates stadium, which is actually closer to Holloway Road.”