I primed them black all over, and then primed the top 75% or so with grey primer.
Then I airbrushed highlights with Vallejo Model Colour Light Grey and then Silver Grey (that's not a metallic color... relax...).
I applied shading with VMC German Grey and then Black. I lined sharp edges in white.
The tiny numerals on each building are waterslide transfers from GinFritter's Gnomish Workshop, via Armorcast.
The weathering was then applied by airbrushing VMC Flat Brown and VMC Olive Grey.
Plants were hot-glued to a couple buildings' bases, and then static grass tufts added to disguise the hot glue.
Windows were quickly rendered hand-painting the area with a solid coat of P3 Coal Black. I then airbrushed the centers with P3 Regal Blue, P3 Cygnar Highlight, and P3 Cygnar Highlight mixed with some White.
I sprayed the same speckled, textured green spray that I used for the table top and the bases of the foliage along the bases of each building. I did not wrap or mask the buildings, allowing the spray to create a natural gradient up the base of each building.
If I wanted to really go to town, I would add overgrowth growing up the sides of the buildings with lichen or coarse turf flock, glued on, sealed, and painted. But I think these will be fine as-is, with a good balance of time and money spent vs aesthetics and playability.
Their biggest downside is that you cannot move INTO them, nor play inside them - You have to go around or climb over. Building kits like those that Warsenal makes are still best for maximum playability.