This is strictly my Traveller universe interpretation...but I went for gray.
Pictured: A Rank 4 Uniform
It's a bit of a hack job (especially the illustration), but I have always been a fan of the uniforms from Star Trek II-VI, so I went and used that uniform as a template for the Imperial officer uniform. The main difference is color, and the fact that the uniforms are gender neutral (i.e. no male and female uniforms). I also attempted to employ a consistent look across each service, since these are Imperial troops and they need to have a distinctive uniform to distinguish them from planetary forces. The Scouts are the exception, in that the uniform is a dress uniform for them rather than a duty uniform.
As for the Merchants, IMTU they are part of the military.
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The Imperial Forces officer uniform provides a consistent look throughout the Imperium. The turtleneck worn beneath the duty jacket is black in color and may be worn without the duty jacket while the officer is off duty. The duty jacket is gray in color, with a black stripe running down one side where the duty jacket closes in the front. A gold Imperial Starburst is worn on the breast. The waistline is accented by a wide, black belt with a simple gold buckle. The uniform is finished off with black pants and boots.
Rank is noted on the sleeves using gold stripes in the following pattern:
Rank 1: no stripes
Rank 2: one 2.54cm stripe
Rank 3: One 2.54cm stripe, one 1.27cm stripe
Rank 4: two 2.54cm stripes
Rank 5: two 2.54cm stripes, one 1.27cm stripe
Rank 6: one 5.08cm stripe*
*The Merchant Service has no rank 6.
On each sleeve above the rank stripes is a 2.54cm stripe in the color of the officer's service branch.
Navy = Yellow
Army = Black
Marines = Maroon
Scouts = Red
Merchants = Green (non-canon)
The stripe on each pant leg follows the color scheme of the stripes on the sleeves.
Scouts:
Members of the Scout Service generally wear the Imperial officer uniform as a dress uniform. Most of the time they wear outfits that are better suited to the Scout Service's exploratory nature. When wearing the officer uniform, to more easily distinguish between a Scout and a Rank 1 officer of another service, the Scout uniform has the logo of the Service attached to each sleeve where the rank stripes normally would be placed.
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If using expanded character generation there are additional ranks besides the six given in the basic Traveller rules. These added ranks are shown below.
Army, Marines, Navy, Scouts (Books 4-6)*:
O1: Rank 1
O2: one 1.27cm stripe
O3: Rank 2
O4: Rank 3
O5: Rank 4
O6: Rank 5
O7: one 5.08cm stripe
O8: one 5.08cm stripe, one 2.54cm stripe
O9: two 5.08cm stripes
O10: two 5.08cm stripes, one 2.54cm stripe
*Scout characters generated using Book 6 do not have rank O10.
Merchants (Book 7):
O0: Rank 1
O1: Rank 1
O2: Rank 2
O3: Rank 3
O4: Rank 4
O5: Rank 5
O6: one 5.08cm stripe
O7: one 5.08cm stripe, one 2.54cm stripe
Enlisted persons for all services wear a simple gray jumpsuit with a single 2.54cm stripe in the color of the enlisted person's service branch. Ranks are noted by a series of diagonal slashes 5.08cm long and 7mm wide located above the service branch stripe. E-1 ratings will have one slash while E-9 ratings will have nine.