Namdhari

Sometimes the color denotes particular groupings within the panth, within the Sikh community.

White tends to represent spirituality and is associated with certain groups, like the Namdharis, who have a living guru, which is not part of the sort of orthodox Sikhism.

Or like the 3HO, the followers of Sikhism of the Western Hemisphere, who are mainly American and European converts to Sikhism, who the men and the women wear pure white, tall, very striking turbans.

And then you'll see a lot of Sikhs historically and sometimes today wearing black turbans.