So we know that the original Dark Angels Legion wore black armour, and this became dark green.

My questions are:
- Do we know exactly when (during Heresy, immediately post-Heresy) the colour change took place? As far as I know it was post-Heresy, but I am not sure of any other sources.
- When was the incident that caused the Deathwing to wear white armour? I'm assuming after M36, as this is when Genestealers were first discovered?

What I'm trying to get at here is figuring out the length of time during which the Deathwing wore green armour.

[runs off to get Deathwing novel...]

It does indeed take place after the first contact with Genestealers. Other than that, I find no chronological signifiers.

The Terminators in the story are recruited from a feral world with a nomadic Native American-style culture, rather than Caliban. After their deaths in glorious battle against the Genestealers, the rest of the company repainted their armour white in imitation of that planet's warrior tradition, recognising the sacrifice that had been made.

The dropship from which the squad deployed was the Deathwing, hence the name of the company thereafter.

Unfortunately its never been made clear when exactly the Dark Angels changed their colour scheme, other than the fact it was some point between the death of Horus and the Second Founding. The Deathwind did indeed change some point after M36, although I don't think the battle recounted in Deathwing is actually the first Imperial contact with Genestealers.

However, I can answer your main question regarding how long the Deathwing wore green armour, because they never wore green in the first place. When the DA adopted green armour, the Deathwing and the Ravenwing retained the old Legion colours, keeping their black armour. Then much later the Deathwing changed to bone-white armour to honour those who saved a world from Genestealer infestation.

I meant it took place AFTER the Ymgarl contact. It was not that contact itself that is described in the novel.

Basically, it places Deathwing post-M36.

Thanks, guys. I've got (two different editions of, actually) the Deathwing anthology which is why I suspected it had to be M36 onwards.

Red Sorceror: Thanks, do you know a source for the Deathwing keeping their black armour?

As I remember the source for the Deathwing wearing black Terminator armour is to be found in the rulebook from the Deathwing expansion for the original Space Hulk.

It contains the story and symbology of the original Deathwing which includes its origins, as well as lots of excellent artwork. ;)

Good stuff; thanks.