yes followers of Yogi Bhajan often wear white (not all of them do, and not all of them are caucasian/white).
from the 3HO website:
As recommended, many wear white clothing, which is a combination of all colors (spin a color wheel and see white)! White enhances the aura and strengthens the magnetic field. Being in this environment offers participants a chance to experience this aspect of the technology. Because of this, you will see a lot of people wearing turbans and white clothing.
ok as to the question, why do many sikh converts wear salwar kameeze (and yes, it is punjabi clothing, and has a lot to do with Sikh history... it was invented by Sikhs so that the women could ride horses into battle like the men! - riding in a sari is very difficult. 
)...
i can only answer for myself. there are several reasons that i prefer punjabi suits. first, it's the most comfortable piece of clothing ever invented. second, it's one of the only pieces of clothing that is comfortable when wearing kashera. third, one is less likely to attract unnecessary attention at the Gurdwara if one dresses appropriately. fourth, when you wear a suit, you always have a dupatta handy to cover your head (if you don't wear a turban or patka), fifth, one can "dress up" for functions in a salwar suit without having to worry about whether one's clothing is too revealing, or wearing a skirt (pantyhose and kashera don't really work well together. ).
i still wear jeans sometimes, but honestly, i prefer to wear suits mainly for comfort reasons.
and i'm one of those non-3HO white converts who wears extremely colourful suits... especially in keseri or any shade of blue. i only own one white suit and i almost never wear it. white gets dirty so easily!
