.Diffuse and diffuse objects are often dry and unpolished . Think of such as newspaper, dirt, and rocks, and is often the base of many materials.Diffuse shading is one of the most common looks in rendering
These objects all reflect light in a wide range of directions from the surface. This means there they won't be shiny or reflective, which requires something like LamaConductor to add shininess and reflectivity.
Color is typically where textures or patterns are connected to create color for opaque objects. This is where a wood color texture would go, for example.
Diffuse roughness is how you would simulate a powdery surface like dried clay or dust. When roughness is 0.0, LamaDiffuse uses the Lambertian model to calculate the diffuse response. When roughness is > 0.0, the Oren Nayar model is used instead.