The second and third I actually took a slightly different approach, but I forgot to photograph them!
Black undercoat > all over with VMC German Grey > base drybrushed & stippled with Rhinox Hide, Mournfang Brown, VGC Sombre Grey, VGC Wolf Grey and VMC German Grey > 'concrete' stippled as base, just less brown.
Basically just threw what I had to hand at it
Razor wire was Balthasar Gold (don't own any Tin Bitz, or whatever replaced it yet) and P3 Quicksilver, Nuln Oil.
I used to find better paints for earthy and 'concretey' colours, but I'll have a look through the VMC range again at some point maybe. Also need a better stippling brush, as the drybrush I have isn't the best shape for it.
The wire actually came out better than I expected - it's just difficult to see when everything is sprayed black!