I recently split an Indomidus box with my son and turned the primaris marines into loyalist Sons of Horus. This kicked off a whole head-cannon of an alternate heresy, which I'm now extending into a chaos faction. In this heresy it was the Lion who became Warmaster and hence the arch-traitor. After his death they become the White Legion.
I did up a Berserker as a test model, with white armour, going with dark green for weapons. Upon reflection I decided to skip the green; 30k era Dark Angels were black and red, green was a post heresy thing. I updated the berserker with black weapons and red accents.
I have a WIP Terminator with this newer scheme. Also went with a chapter symbol which is a derivation of the Dark Angels symbol, echoing what happened with the Sons of Horus -> Black Legion iconography shift.
Also working on a Daemon Prince - Holguin the Desecrator, bearer of the Veridian Blade.