[Pontiac]

the Klansmen offered Nation members the right to wear “purple robes” as Klan partners

You would think that the members of the KKK would know the significance of "Purple Robes."

In the Middle Ages only kings could wear purple.

The tradition of purple robes signifying Kingship has been carried down through the years in numerous Christian religious ceremonies. It is well known.

The KKK would be signifying that the Black Nation of Islam was dominant over the White KKK.

[Hieronymus]

True long before the Middle Ages.

That said, with Christianity, purple for a variety of reasons became a liturgically penitential colour, and white trumps purple.