Children of the Emperor! Death to his Foes!: Picking a 30K Army?

Here's a short summary of the Horus Heresy legions, to help newcomers pick an army!

Legion I: Dark Angels

Primarch: Lion El'Johnson
30K Colors: Black
40K Colors: Dark Green
Summary: Knights with mysterious loyalties.

Legion II: Unknown

Legion III: Emperor's Children

Primarch: Fulgrim
30K Colors: Purple
40K Colors: Magenta
Loyalist or Traitor: Traitor (Slaanesh)
Summary: Previously elitist aristocracy. Later lurid hedonists.

Legion IV: Iron Warriors

Primarch: Perturabo
30K Colors: Black and Yellow
40K Colors: Black and Yellow
Loyalist or Traitor: Traitor
Summary: Siege specialists.

Legion V: White Scars

Primarch: Jaghatai Khan
30K Colors: White and Red
40K Colors: White and Red
Loyalist or Traitor: Loyalist
Summary: Mongolian bikers.

Legion VI: Space Wolves

Primarch: Leman Russ
30K Colors: Grey
40K Colors: Slate Blue
Loyalist or Traitor: Loyalist
Summary: Viking Werewolves.

Legion VII: Imperial Fists

Primarch: Rogal Dorn
30K Colors: Yellow
40K Colors: Yellow
Loyalist or Traitor: Loyalist
Summary: Faithful Guardians.

Legion VIII: Night Lords

Primarch: Konrad Curze
30K Colors: Dark Blue and Red
40K Colors: Dark Blue and Red
Loyalist or Traitor: Traitor
Summary: Psychopathic Batmen.

Legion IX: Blood Angels

Primarch: Sanguinius
30K Colors: Red
40K Colors: Red
Loyalist or Traitor: Loyalist
Summary: Angelic Vampires.

Legion X: Iron Hands

Primarch: Ferrus Manus
30K Colors: Black
40K Colors: Black
Loyalist or Traitor: Loyalist
Summary: Grim Cyborgs.

Legion XI: Unknown

Legion XII: World Eaters

Primarch: Angron
30K Colors: White and Blue
40K Colors: Red and Gold
Loyalist or Traitor: Traitor (Khorne)
Summary: Blood thirsty berserkers.
How to Paint

Legion XIII: Ultramarines

Primarch: Roboute Guilliman
30K Colors: Blue
40K Colors: Blue
Loyalist or Traitor: Loyalist
Summary: Disciplined Strategists.
How to Paint

Legion XIV: Death Guard

Primarch: Mortarion
30K Colors: White and Green
40K Colors: White and Green
Loyalist or Traitor: Traitor (Nurgle)
Summary: Previously stalwart footsoldiers. Later diseased plague bearers.
How to Paint

Legion XV: Thousand Sons

Primarch: Magnus the Red
30K Colors: Red
40K Colors: Blue and Gold
Loyalist or Traitor: Traitor
Summary: Egyptian wizards.

Legion XVI: Sons of Horus

Primarch: Horus
30K Colors: Sea Green
40K Colors: Black
Loyalist or Traitor: Traitor
Summary: Urban gang members. Leaders of the Heresy.
How to Paint

Legion XVII: Word Bearers

Primarch: Lorgar
30K Colors: Dark Red
40K Colors: Dark Red
Loyalist or Traitor: Traitor
Summary: Religious zealots.
How to Paint

Legion XVIII: Salamanders

Primarch: Vulkan
30K Colors: Green
40K Colors: Green
Loyalist or Traitor: Loyalist
Summary: Coal skinned optimists from volcanic planet.

Legion XIX: Raven Guard