How do I assign a material to an Archipack object? I use Blender 2.79 and am still pretty much a beginner.

I tried both Archipack and Archimesh. Archipack seems a bit more comfortable but I don't know how to assign a Material to it or change it from the default grey to something different. In Archimesh that is easy. I would like to use the Principle BSDF Shader and Cycles.

I cannot find documentation on either of those addons either... just a long thread in a forum.
Archipack can take advantage of a predefined material library.
If the default material library is missing, you get dumb white materials assigned.
Please take a look at https://github.com/s-leger/archipack/wiki/Materials in order to setup your default material library.
The aswer of @light-pankakes remains still valid when it comes to setup new materials.