The options for the Category property on media objects, the Colors property, and Topics on weblinks are not customizable by design.

We are not planning to add custom support.

That said, these properties work well together with collections and queries:

• If you’ve been using collections to categorize content, you can now let Category and Topic properties do that work automatically using queries. For example, instead of maintaining a collection for “all images of handwritten notes”, you can create a query on Images filtered by the Category property.
• Collections are still useful when you want to manually curate items (for example, a “My favorite pictures” collection). But for rules-based groups like “all images of handwritten notes” or “all weblinks about a topic”, queries based on Category/Topic can replace those collections and keep your setup cleaner.