Sketchfab scene uses all sorts of "nodes" , they treat them more like entities with types. SO it is not only for materials as in blender
The viewer API has functions to retrieve the nodes or the materials individually.
You set colors and change textures on materials
You hide/show or translate nodes that have a "matrixtransform" type.
That is about it. While it is possible to perform hide and show on nodes with a different type , you cant move them for example.
You should check out my Viewer API Utility , it makes using the Viewer API easier and of coarse if you have coding skills you can look at the code as well.
I for example filter the nodes I list to the ones with a type of "matrixtransform" .. since they are the most useful. This filtered list will represent all the individual objects in the scene.
For Materials I retrieve the list individually as well