David Ely - Dark Angels Painting "Mistakes"

Something I enjoy about the Warhammer hobby is treating the factions like real, historical armies that I’m recreating. Just as an actual military force has specific uniforms and markings worn for different fields of battle, so do Space Marines, Guard regiments, and Sororitas Orders. With that in mind, here are a few things I occasionally see people get “wrong” when painting and modeling Dark Angels. As always, paint your own models however you want. If you want bone helmets, nothing’s stopping you. This is just what GW’s studio models and artwork tend to prescribe.

Chest insignia should be bone white, not gold, for greenwing and Ravenwing. On Deathwing, they’re green (or sometimes red if they’re wearing a green robe, to break up the colors). This provides a consistent color palette across your army, giving a touch of bone white to the greenwing which is inverted on the Terminators. The Dark Angels chapter symbol, and the Ravenwing variant, are cold white (often actually painted a light grey so you can highlight with pure white).
Shoulder trim should be green on most troops. Some officers get gold trim, based on your whims, as can Librarians and Chaplains. Ravenwing bikers tend to have black trim. Power armoured Deathwing can have gold trim. The theme across these two bullet points is that gold is actually used sparingly for Dark Angels. That said, giving the occasional model something to differentiate himself is totally fine.
Helmet colors should always match the model’s armor. Dark Angels don’t pick out their sergeants' or lieutenants' helmets in any particular way, aside from perhaps a skull or sword on the forehead. A veteran sergeant might wear a robe.
Hoods look best when the model has some other cloth on him. There’s no rule about who can wear a hood but it’s another way to decorate a sergeant.
Winged helmets are for Company Masters (Captains) and Champions. Some Deathwing Terminators have wing ornaments on their armor, too. (30K Dark Angels used them more widely.)
Winged models are a Blood Angels thing, since their Primarch actually had wings. Dark Angels' iconography has wings, some equipment like the Ravenwing bikes have winged elements, but the battle brothers themselves are never depicted as being winged. The closest anyone comes are small wing adornments on the Lion’s power pack.
Robes, not tabards, on veteran sergeants, Lieutenants, Masters, and other Inner Circle members are very Dark Angels. Tabards and pteruges are best left to Ultramarines, Imperial Fists, and Black Templars. The distinction might seem small, but think of 40K Dark Angels as warrior monks, not knights. The further they move up the ranks, the more they cover up.
Censors are a common decoration, but rarely candles.
Boltgun grips are black while the rest of the casing is red. I don’t know why but it’s something GW’s studio models do.
Ravenwing models should only ever be those that get the Ravenwing keyword, not just any attack models. More on that here.

As a final word, knowing when to tastefully break these “rules” can give your models a ton of character. Maybe one particular battle brother has gold script along his shoulder trim. Another has a checker pattern on his helmet. These little touches make it seem like each marine has his own story. Some chapters, like Blood Angels, ornament their armor more than Dark Angels, who prefer to be a little more staid, but there’s still plenty of room to make them not all look like they’re fresh off the factory line.