Which northern Saudi desert is noted for red sand dunes?

The northern Saudi desert noted for its red sand dunes is An Nafud.

An Nafud, also called the Great Nafud or Nafud al-Kabir, is a large erg in north-central Saudi Arabia. Its sands have a distinctive brick-red to reddish appearance, a color associated with iron-rich material. Strong winds shape the desert into broad, crescent-like dunes separated by rocky plateaus and lower ground.

The desert occupies a great oval depression and covers roughly 65,000 square kilometres. Rain is infrequent, but occasional winter precipitation can produce short-lived grasses. Those seasonal pastures have historically supported Bedouin herding, while oases near the desert have allowed cultivation of dates, grains and other crops.

An Nafud is often confused with Ad-Dahna because the two are physically connected. Ad-Dahna is the long, narrow sandy corridor extending southward from An Nafud toward the Rub' al Khali; it is not the northern desert itself. The name also appears in variant forms such as Nafud, An-Nafud and Nafud al-Kabir.