Well...
Depending on what you think is MOST current or canon (I prefer to use the Index Astartes article about them)...
All Veteran Raven Guard Marines have white shoulder pads and arms.
I think Veteran Sergeants also have White Helmets.
So, I'd paint the Terminators with white helmets and white shoulder pads and arms.
For the Dreadnought, I'd go with white "arms".
In fact, I'm pretty sure there IS a picture of a Raven Guard Dreadnought in either this codex or the
SM codex from last edition...
Anyway, that's how I'm planning on painting them.
Here's how someone else did their Terminators: