It depends on how you expose the attribute in your OData.
You can setup if you want to use the String or Enumeration data type. When you use string, you can then filter just using the key value as string:
If you configure it to be an enumeration, it's indeed like you describe:
where namespace (Core) is defined in your OData general settings and the exposed name and values of the Enumeration can be altered on the tab 'Enumerations'