...

Here is an example of how to set a color user attribute with an OpScript:

While all user attributes can be set up as a GroupAttribute, color attributes are the only type that requires it.  Int, float, and string user attributes can be set directly, with their type inferred from the Katana attribute type:

Using User Attributes

RenderMan ships with several plugins that make use of user attributes.  PxrAttribute or PxrVary are patterns that read user attributes. The PxrCryptomatte sample filter can create different outputs based on string user attributes.  When using these plugins, the parameter for the attribute should include the prefix user: followed by the attribute name.