Here's the response I got back from Gurtej Singh in Chandigarh, the National Professor of Sikhism, about the SGPC colours:
"Most of the people who work for SGPC were recruited by the Akalis. They being Akalis themselves wore white dress and blue turbans. This became the uniform. After Baba Jarnail Singh yellow turban and flowing robes became current and respectable. White turbans became somewhat respectable when Baba92s became important in Sikhs socio-political life and when Bibi Jagir Kaur became the President. This is the reason for these colors being current in the SGPC."