While working on my Nightshade Loadout Guide, I got curious whether having a certain cosmetic item can be an advantage or a disadvantage in darkness. So I took a few screenshots.

Notes:

All of the screenshots in this article are taken at my Suggested Settings.
You can put the mouse over a screenshot to see the name of the cosmetic armor equipped, and you can click on a screenshot to enlarge it.
Screenshots with Standard armor also use the default helmet. Other screenshots have a random helmet just to demonstrate that some of the cosmetic helmets also have glowing elements.

Terran Republic

Standard and Composite Armors have prominent glowing elements on the waist, back and feet.
Gold Trim has all of the same plus some additional glowing from the gold elements, especially on the shins.
Pathfinder is okay from the back, but it has the same glowing elements on the front side as the Standard Armor.
Havoc offers the best concealment, only having glowing elements on the feet.
AVA has a lot of glowing elements.

New Conglomerate

Standard and Composite Armor are the least visible. Sigma is just as good.
Gold Trim does not glow by itself, but it can sparkle and reflect light, potentially drawing eyes to your position.
Pathfinder has a large glowing element on the backpack, and Nomad Armor has a couple of glowing elements around your waist.
Important to note that the default helmet has glowing elements on the back. Replace if possible.

Don’t be worried about bright yellow lines – they are not bright in third person:

Vanu Sovereignty

Standard and Composite Armor are least visible. There are a few glowing elements on the back, but they are not visible outside of close range. Pathfinder is almost as good.
Darkstar has a couple of glowing elements on the knees.
Other armors have clearly distinguishable glowing elements that make the player easily visible in the dark, with Lumifiber and Auraxium being the worst.
Important to note that default helmet has a clearly visible glowing triangular element on the faceplate. Replacing the helmet is highly recommended.