Alpha Legion: Painting 1 - Legion Indigo

Let's talk Alpha Legion armor color. 

This old Index Astartes article shows two different ones: the purple and green scheme and the blue and green scheme. Seems like GW has gone for the blue and green scheme as the official Alpha Legion scheme in the CSM Codex. The old purple one doesn't seem to be very popular in general, but I see the blue and green around all the time.

Index Astartes figure showing pre- and post-Heresy schemes.

Alpha Legion: Background 1 - History

If you've heard of the Alpha Legion, you've no doubt heard they are secretive, duplicitous, and contradictory to the extreme. The Horus Heresy (HH) book series has done much to improve and expand on previous background material that merely stated, not showed, the tactics and methods the Alpha Legion use to achieve their goals. The HH books have also provided some inkling for the basis of the Alpha Legion's goals, while steering clear of stating anything explicitly (as well they should!). And as much as I like having the answers, keeping the Alpha Legion's goals a mystery fulfills the very practical purpose of letting fans of the game and story, like me, make up their own reasons, and therefore decide on the motivations and goals of their own cells of Alpha Legionnaires.