You cannot create smart collections within smart collections. However, you can create collection sets. If you created a collection set called "portfolio Images" and then in that set create a smart collection for red labels, one for green labels, one for blue labels, etc., then when you choose the collection set it will show the aggregation of all its contained smart collections. (If you don't see that check that "Include Photos from Subfolders" is on. You can access that from the Library menu.)