User attributes are a convenient way to vary shading across multiple objects without changing a shading network. They are especially useful as they can vary across instances, unlike primvars. Here is an example of how to set a color user attribute with an OpScript:
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Color attributes are the only type that require a group attribute. Int, float, and string user attributes can be set directly, with their type inferred from the Katana attribute type:
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RenderMan ships with several plugins that make use of user attributes. PxrAttribute, PxrVary(?), PxrVariable(?), PxrCryptomatte (string attribtues used by the sample filter). When using these plugins, use the prefix user:
followed by the attribute name.