Brown

Walnut was a dark shade of brown named after a certain nut.

The War Doctor wore a double-breasted trenchcoat of walnut leather when he, Fey and the Sisterhood of Karn fought Mark VIII Daleks and the Morlontoa of the Seventh Sky on the Dorian Nexus. (COMIC: The Clockwise War)

The Tenth Doctor wore a walnut brown tie with stylised peacock-eye steel blue and brown motif in honeycomb grid when he helps Elton Pope and meets an Abzorbaloff, (TV: Love And Monsters) during the Battle of Canary Wharf involving Mark VII Daleks including the Cult of Skaro versus Cybus Cybermen including Cyber-Leader One and Cyber-Leader Two as well as the fall of Torchwood One including Yvonne Hartman, (TV: Army of Ghosts|Doomsday) when recruiting Martha Jones after the Royal Hope Incident, (TV: Smith and Jones) when he and Martha join forces with William Shakespeare to defeat Carrionites on Southwark during 1599, (TV: The Shakespeare Code) when saying goodbye to the Face of Boe after helping him save New New York from Macra during 5,000,000,053, (TV: Gridlock) when meeting an older Tim Latimer during an Armistice Day, (TV: The Family of Blood) during an encounter with Pyroviles and Pompeii's volcanic end on 79 AD, (TV: The Fires of Pompeii) when he and Donna Noble encounter a Time Beetle from the Trickster's Brigade, (TV: Turn Left) during the Planetary Relocation Incident involving Davros and the New Dalek Empire as well as his siphoned regeneration which spawned the Meta-Crisis Doctor, (TV: The Stolen Earth|Journey’s End), and when he met Christina de Souza and space stingrays. (TV: Planet Of The Dead)