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
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
Primarch: Corax
30K Colors: Black
40K Colors: Black
Loyalist or Traitor: Loyalist
Summary: Stealthy strike force.
Legion XX: Alpha Legion
Primarch: Alpharius / Omegon
30K Colors: Blue Green
40K Colors: Blue Green
Loyalist or Traitor: Traitor
Summary: Deceptive masterminds.