[History] The Astra Militarum cohort Angelis-CC were known to paint bullets blue, believing that the colour was sacred to their dualistic interpretation of the God-Emperor. Under their belief system, the two heads of the Imperial Aquilia represented two independent brains and entities, sometimes acting alone and sometimes acting as one. Their regiment was renowned for brutality, wielding improvised human-scale boltguns and handmade rounds. The cohort perished to a man three hundred years ago during the Skullzar Crusade, and Angelis was subjected to Exterminatus due to an Ork infestation one hundred years ago. 

... Sorry. I don't think that is particularly relevant. Is it?

[Occult Studies] None of your business.
[Criminology] Maybe there is a ritual significance? Even if the specifics are forbidden knowledge, sometimes cultists express themselves with unified paint schemes.
[Art History] Then why is the door paint green? Because now that I'm looking at it in more detail, I can confirm this is [i]exactly [/i]the correct shade of green paint for a member of the Dark Angels Chapter. The blue isn't a codex compliant hue though.
[Human Terrain] And there is a Dark Angel in the Factory Cathedral. So much of this could only have been done by an Astartes, but...
[Bullshit Detector] But on the essential point, you're right. This feels... wrong. Wrong and [i]stupid[/i]. Like someone fucked up a frame job. I think there are two stories here: What was meant to happen, and what actually did.