To accomplish a suitably barren and tortured environment, paint the exposed areas of bedrock, boulders, and scree with a drab colour scheme of Scorched Brown, Graveyard Earth, and Kommando Khaki.

Overbrush the Chaos Black undercoat with Scorched Brown. Next, liberally drybrush over this first layer with Graveyard Earth. Finally, give everything a light drybrush of Kommando Khaki (make sure you pay particular attention to the boulders).

Even though they are going to become Shelob's webs, give the areas of melted foam the same treatment as above. This step will ensure that the white you apply later to the webbing will not become too glaring or stark.

To paint Shelob's webs, you have to build on the painting you did before. The Kommando Khaki you applied will make a nice base for the white on the webbing.

To make the webs stand out from the surrounding stone, heavily drybrush them with Kommando Khaki once more. Only focus on the bits of foam that look like web.

Once this layer has dried, drybrush the same areas lightly with Skull White. This layer must be applied with care as too much pressure or paint will cause the whiteness of the webs to be too bright.

Retouch any Orc debris (or other bitz you used) with Chaos Black. Then, depending on what material the debris is made of, paint it with the following schemes.

Metal
Start with Boltgun Metal, then wash it with a mixture of Black Ink and Chestnut Ink.

Bow and Leather Shield
Basecoat with Scorched Brown.

Weapon Hafts
Basecoat with Chaos Black.

Leathery Dead Orc Skin
Don't even touch it! The drybrushing you did was enough. It looks just like Shelob sucked out his lifeblood!

When you are done drybrushing and painting, you can clean up the flat areas above the caves and on the table sides with flat black house paint. Painting these areas black keeps the focus on the tunnels and also represents areas of solid rock.