OK - the full history of the Dark Angel change to green was as follows: (This is actual history not in game history)
Back in the day when
GW were first releasing lots of Games they hired a load of artists who werent necessarily into the hobby nor knew anything about it. One of these, Jim Burns
http://www.alisoneldred.com/artistJimBurns.html was hired for the cover of the original Epic game "Space Marine" an Expansion / Update of Adeptus Titanicus what people know as Epic 1st Edition - It was this game that INVENTED the Heresy. (The needed a story reason as to why you used the same models on both sides - a Civil War storyline was created by Jervis and Rick)
https://cf.geekdo-images.com/images/pic805134_md.jpg
He didnt know
GW stuff at all well, but was a world renown artist, and was asked to paint a picture of Space Marines, and for this he was given pictures of Salamanders, and the iconography section of the Rogue Trader book.
He took the Salamander colours and used them on the Dark Angels not knowing that each chapter had unique colours.
However the art is amazing (as all his stuff is) and since this was the late Eighties (1989 to be precise) they went with it, especially considering that most of the Background that we know and love hadnt even been written - or was in the process of being written. By the time 2nd edition dropped -
DA had been green for years
The other reasons - Easier to Photo, Army looks better green etc are all after the fact changes.
The Deathwing being Bone White is another example how Story Retcons and artists created stuff in a vacuum in the early days. Bill KIng who wrote the short story Deathwing that told of Two Heads Talking decided that the Dark Angel homeworld was an Apache Indian style homeworld. This has since been retconned into being one of the
DA Recruitment worlds since the fall.
But thats why the Deathwing have Feathers on everything.
Oh and how do I know this? I worked for
GW around the time that a lot of this happened and a short while afterwards, so I got them from the horses mouth - I even ran the museum and had to tell these stories to the kids as I showed them round.