Hi there,
I'm no expert on the
Marine-Brigade Erhardt but here's what I know:
The members of the Marine-Brigade Erhardt carried regular field grey army uniforms but with their own insignias, etc. On some photos they wear a star with an anchor as collar patches, similar to the star of the guard units of the imperial army (maybe to mark their self-proclaimed elite status, maybe only some members of the brigade wore them, like stormtrooper groups/units and such).
In early 1919 they also adopted a specific patch as part of their uniform in form of a viking ship framed by a rope and the name 'Wilhelmshaven' written below the ship. After the brigade was disbanded its former members continued to carry a modified version of this patch (the name 'Wilhelmshaven' was replaced by 'Erhardt') as a sign of honour/affiliation.
Here are photos of the two versions of the patch after a quick google search:
original version (edited for a better photo):
Modified version after disbandment:
Of course they also carried other insignias often visible on photos like the swastikas painted on their steel helmets and on the side of their lorries, etc.
I think there are two reasons for the field grey uniforms:
1. It was common practise during the Great War for Marine-Divisionen serving in infantry role on the western front to start the war with their original naval/sailor uniforms but to adopt the field grey ones during the course of the war. Often only retaining the rank insignias of the navy and sometimes their sailor hats instead of the field caps of the army (in action they also wore steel helmets of course to blur the differences even further).
2. Maybe even more important, after the Naval Mutiny that had started the revolution, the sailor had become a real icon maybe THE icon of the revolution. No one fighting for the 'White' cause would have wanted to be mistaken for a 'Red'/'Spartakist'.