Sikhism was a religion primarily associated with India. Its adherents were called Sikhs. According to Ram Singh, there were 30 million Sikhs on Earth in 2016. He felt his religion was not bleak.
They believed that the important thing in life was to do good action. The thinking was that, if one did a good action, somewhere in the process there had to be God. They wore karas to remind themselves that they were part of a community, and that their hands and lives should go to good work. (TV: Brave-ish Heart)
Varun Singh, who practised Kesh, had a beard and wore a turban. (TV: For Tonight We Might Die)
They typically wore turbans, letting their hair grow long in a practice called Kesh. (TV: For Tonight We Might Die, The Coach with the Dragon Tattoo, Brave-ish Heart, COMIC: The Crystal Throne) A kangha was an object important to their culture. (TV: Brave-ish Heart)
Ram Singh and his father Varun were Sikhs, but April MacLean noted that he wore his hair short. (TV: Brave-ish Heart) While Varun wore a turban, (TV: For Tonight We Might Die, The Coach with the Dragon Tattoo, et al.) which Ram appreciated as it reminded him of where he came from, Ram himself felt he needed to look "sharp" in "modern times". He also Ram pointed out, "Ever try going through airport security in a turban?" His dad didn't like that Ram did not practice Kesh. Ram still had a kangha that his father gave him. (TV: Brave-ish Heart)
The taxi drivers who drove the Seventh Doctor and Ace from Heathrow Airport to Smithwood Manor were Sikhs. (PROSE: Cat's Cradle: Warhead)
Donna Noble's cousin Janice was married to a Sikh. (PROSE: Ghosts of India)
Ajeeth, servant of Lady Cornelia Basildon-Stone, was a Sikh. (COMIC: The Crystal Throne)