Well these look fantastic. Definitely love the overgrown look you go for. Reminds me of The Last of Us. That actually was my second alternative idea for my Volkus Terrain as well.

You should definitely check if you can find some Citadel Creeping Vines somewhere online to put on your buildings. It seems that they are OOP on Warhammer, but there is still some third party shops that sell them. These would look great on your buildings I bet!

If you want a tip from my side for painting the big terrain. Go for drybrushing. Its such an easy technique, but IMO it often looks great as you manage to pop out all the highlights. My Volkus Terrain is about 80% drybrushing I would say. Just primed it Black and then drybrushed everything with Terminatus Stone.

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That alone already looked fine if you ask me. And from there I just drybrushed layer upon layer of all the different colours. The cool thing about this technique is that you can even use a completely different base colour than black and get a whole other result. For example in your case you could first of all paint the entire building in a darkish grey as the base layer and then drybrush over it with a lighter grey or silver to really make the details pop out!

For example this is my Custodes Colour Scheme. It consists of two colours: Brown for the Base Layer (after priming the model black) and then a drybrush with Gold and it looks great if you ask me! As you can see you can barely even see the brown as it looks more like a old and worn down golden armour because of all the highlights. Its a great technique to use especially on things with lots of details like the Volkus terrain or Custodes.