Terrain update: Painting the details on the Realm of Battle Cityscape boards

I am really pleased with the choice of Straken Green for the storage tanks, and the Mournfang Brown wash
for the rust effect

Summer is meant to be terrain painting season, but I have been focusing on building up the both the Ultramarines' and Iron Warriors' tactical and terminator squads.  I have, however, made a start on painting some of the details on the Realm of Battle Cityscape boards (ammo boxes, storage tanks and the like).  These are a devil to paint: a 60cm x 60cm board is not something you can hold in one hand!  I have painted the details on 3 boards now and the others have fewer, so they should be much quicker. I still need to drybrush a highlight on the details (Probably with Screaming Skull).  I am itching to paint a mud wash on the boards in patches, but I'm not sure yet what colours to use (maybe Doombull brown and another wash with Steel Legion drab).  If anyone has any suggestions on what colours to use and how to make up the washes, then please put them in the comments section.

I still need to drybrush a highlight on the ammo and fuel cans.  Brown streaks on the concrete will have to wait
until next summer