Material layering in RenderMan for Houdini is incompatible with other RenderMan bridge products.
In RenderMan for Houdini, we layer the material parameters when its local parameter values have changed from the default values.
In this example, we set a green diffuse color in PxrSurface. Then we layer a PxrLayer node on top that has a cyan diffuse color.
If you want to force the default value of the layer to override the layer underneath, you can lock the parameter. THis is a workaround since Houdini currently does not provide an option to force the value to be local.