So far, there's been a few legions revealed, so let's go over them and what we know about each:
Star Defenders:
Blue and Red color scheme.
Symbol: A pair of downward-facing chevrons.
Seem to be the analogue for Ultramarines.
The Captain's from this one and most of the group's misadventures seem to circle around this one as a result.
Flame of Space King:
Orange and yellow color scheme.
Symbol: Orange flame on a yellow background.
Their armor seems to have big "eyes" on the exterior armor plating, suggesting some kind of night vision apparatus.
Seems to be the Iron Hands stand-in.
Bryce and the heretical Imaginator from the intro of Episode 3 come from this legion.
Beholders:
Blue and silver color scheme.
Symbol: An "X" made of linked chains.
Appears to be the analogue to the Black Templars.
Hatemonger's legion. Defined by their hatred of other factions and their zealous adherence to what they believe to be Space King's doctrines.
Galavanters:
Purple, gray, and white color scheme.
Symbol: A lance with a pennant.
Seems to be the equivalent of the Knights Errant or Mentors.
Terry's legion. Generally pretty upbeat dudes in all regards.
Ostragoths:
Black and orange color scheme.
No Symbol shown.
Appears to be the analogue for the Space Wolves. Appeared early in Episode 2. We only know the legion name because it came up during an interview.
The Ostragoth here is also the leader for a different Reclaimer Squad, hoping to find more Space King Artifacts than the Captain's and ergo prove themselves the best.
Wardens:
Dark Red and White color scheme.
Symbol: A skull ringed in green.
Very clearly nods towards the Dark Angels with their cool robes.
Seems to be one of the more intellectual Legions. Had green laser swords, suggesting that even though they knew what the crystals were for, they still couldn't help but use a few of them to make their lasers green.
Coagulators:
White and blue color scheme.
Symbol: Blue Diamond.
Their scheme is very clearly a nod towards the pre-heresy World Eaters, but their culture is more a nod to the Blood Angels.
Oddly obsessed with Blood. Uniquely, got massacred by Chestnut (of all people) in a Hatemace-fueled super-homicide.