Historically, the uniforms of the Merchant Marine have varied depending on the particular shipping company, time period, and type of Merchant vessel.
Right: 22nd century Merchant vessel bridge
Until the early twenty third century, there were no standard merchant uniforms and most merchant officers and crewman simply wore civilian clothing during the course of their duties.
It was not until the year 2245 that the Merchant Marine began publishing standard uniform regulations and then only for vessels and crews under direct contract with the Federation Department of Transportation or attached to the Starfleet Auxiliary Fleet. Planetary shipping companies, as well as several private cargo and freight companies, have often neglected any established uniforms, allowing their crews to wear practical clothing as desired.