Since the release of the new Codex I've been trawling the Internet for the slightest hint of legion-specific rules. As it turns out, it doesn't look like there are any so I'm just going to paint mine as all different legions, starting with... DEATH GUARD :D Now you may wonder why I've gone for Death Guard when I love the Thousand Sons but I just want an excuse to use more of my rust effect paint which wouldn't exactly work well with the bright and shiny look of the Thousand Sons.
I've been looking into their colour scheme and while most pictures show green plague marines I think they'd look much more striking if they are wearing their pre-heresy colours, but weathered and looking like they've really taken a beating over the last 10,000 years. As a result I've decided that their armour will be white and they will have the same pre-heresy marks on their armour (chapter etc) but over the top will be the Nurgle mark and lots of rust, bullet holes and strike markings so as with the Plaguebearers I did in my Daemon army, the focus will be the detail. I'll experiment first but I'm pretty sure it will look good.
I do plan on painting my cultists as just ordinary people though, just so that they'll fit into any legion really. My Helbrute will probably be Death Guard as well but I'm not sure just yet.