It's hard to explain.
I use a soft brush at low opacity.
I paint that on a multiply layer for the shadows.
I use a low-t0-mid warm grey as the color.
Sometimes i will use several multiply layers to build up.
Each will be at differing opacities.
Just lots of subtle adding here and there to build a shape.
It's gets balanced on the light side with highlights, so your base color needs to something at least off of white.
Tan to cream, or darker. Then the highlights will show up well.
I tend to do my highlights as a softlight layer, rather than overlay.
I sort of just used the caterpillars to get the main shape of the ridges and sub-ridges.
I then realized it just didn't really work with those lines in there and turned that layer off.