Obviously all models start with a grey base...( I used grey spray... thinking about it now though I might have wanted to start with a black base coat and dry brushed all the grey on to give the cracks and seams more definition. Live and learn.)
The chest wings are brass with chainmail highlights (skulls are gem settings) and hoses, belts, pouches and holsters are chaos black. The occasional Beakie has chainmail power couplings across his chest. Most additional detail (such as skulls elsewhere or wing or laurel detail) is brass colored.
The helmet grill on Mk7 armor is chainmail with black hoses leading to it and the ridge crest is also chainmail (except assault marines where the ridge crest is brass)
The right shoulder pad in filled with elf grey (obviously lighter than the dark grey of the rest...) the squad's type Marking (Tac= up arrow, Dev=up chevron, Assault=x etc.) is marked in dark green (actually using ork wash for this) with the company number (in roman numerals) over this in black. Having set the company numbers on the Shoulder pads with decals, I leave the area directly under the number unpainted and this lets the number show very well.
The Left shoulder pad is left barren (except for 2nd Squad, 2nd Company [Stud Squad] that I have created studded shoulder pads for) to show that the Grey Legion are dishonored and without Chapter insignia.
Each squad is given some form of honor device that links them and shows squad affiliation. Such as 2/2 above with the studded pads and 1/2 (1st squad/2nd Co.) with skulls painted over their faces [Skull squad] and 10/2 (a dev squad) with solid chainmail painted helmets (brass offset ridge and grill) [the Metalheads]
Many Grey Legion have adapted weapons or equipment from fallen foes and the Legion has many Marines with bits of armor stolen from Chaos enemies and incorporated as symbols of triumph into their own armor. It is not uncommon to see a Chaos shoulder pad or even helmet on a Grey Legion Marine. These trophies are accepted and at times even encouraged. Some Marines carry other grisly trophies such as enemy alien heads or the pelts or skins of alien beasts. (I like to personalize many if not most of my marines with arms, pads, helmets and other gear from Chaos and other sprues to give them each individuality.)
All Character troops ( chaplains, apothecary, and so on...) are based in grey with the appropriate color (chaplain black, librarian blue and so on...) on the torso, shoulder pads, and right grieve of the model. Captains and other officers are also based grey with the right shoulder pad and right grieve being given over to personal heraldry. Most Characters are also lined with metallic trim ( silver and brass for lesser characters and gold for Higher ranking ones.) Chaplains (as well as deacons) also have black painted helmets and usually gloves as well.
Vehicles get the full treatment as well. Grey Primer with Black doors and boxes. Silver drybrushed weapons. Small elf grey sections (different with each vehicle type) with the company number in Roman Numerals on each vehicle.
All my models and vehicles except my land raider and Terminators (and the Scouts of course) are painted to be from the 2nd Company. The Termies and land Raider obviously from the 1st and the Scouts from the 1st (if veteran) or 10th (if regular.)