A bit of googling seems to indicate that it's a black helm with a white stripe, with white shoulder pads/arms, although one person seems to have done a cool variant where the front part of the helm is white and the rest black:
Chapter Colours
The Raven Guard Chapter wears black-coloured Power Armour. Their Chapter iconography consists of a large white raven painted on their left shoulder plate. The left shoulder plate's trim is black. The right shoulder plate's symbol and trim colour denotes which type of squad the Astartes serves in. Green trim indicates a Raven Guard Space Marine is a member of a Tactical Squad, red an Assault Squad, yellow a Devastator Squad and black trim is worn by Veterans. Some Raven Guard Force Commanders or high-ranking Veteran officers and Sergeants have all-white shoulder plates, as well as helmets and sometimes the armour covering their entire arms.

Link to the above,
HERE.
Sources: Google (obviously)
http://warhammer40k.wikia.com/wiki/Raven_Guard
http://wh40k.lexicanum.com/wiki/Raven_Guard