The white command beret was a special part of the uniform for command level officers of the Royal Manticoran Navy. Manticoran tradition was that officers currently in command of a hyper-capable starship wore a white, rather than black, beret.
It was considered bad luck for an RMN officer to replace the first white beret they were given upon receiving their first hyper-capable command. (HH3)
Since its use was restricted to the commanders of hyper-capable ships, flag officers did not wear it. The only exception was the Captain of HMS Unconquered, a ship maintained in continuous commission for historical reasons; that Captain was selected by the Monarch from a list approved by vote of all active RMN officers. (HH11)
As a sign of respect, when visiting another hyper-capable ship, a captain did not wear the white beret, keeping it instead tucked under a shoulder epaulet. (SI2)