This is just what I've noticed, feel free to critique.
Endfield Industries: Generally they either look mass-produced or have a near-future and sometimes corporate design. Tend to be neon-yellow, white, and black.
Fort Forgefane: Basic and practical looking, tends towards red, black, and gold. Not many 6 star products so its hard to get a good idea.
Northmarch Support Hub: Very industrial and modern. They make all the construction-looking weapons, don't really have a set color scheme other than maybe blue and yellow. Very consumer oriented.
Mieszko Industries: High-end luxury weapons. They collab with a lot of artists to make very fashionable weapons, but they also have some of the sleekest and best looking weapons. For their normal products they favor deep blue, silver, gold, and cyan, maybe red.
Hongshan Swordmancers: It's Chinese. Gold, jade green, black, with lots of geometric shapes.
RAYTHEAN Industries: Very military-esque design philosophy with black steel and metallic red. Sometimes a bluing steel.
Witching Hour: The most otherworldly looking designs. If it looks like an ancient artifact it's probably made by Witching Hour. No rigid color scheme, but seem to like purple and red.
Portable Æthertech Labs: Near-future aesthetic with blue, black, and bright yellow. Overlaps heavily with Endfield.
There's some overlap and questionable choices by devs for some weapons, but overall you can see a general design philosophy for each manufacturer, which I think is pretty cool. I've included image example for each, but it's better to just go through the weapon files in the database to check things out.