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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.
Diffuse Color Override Example
Emit Default Values
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.