Items available in various sizes and colors are collectively called a family of items. These grouped Items have a More options drop down button beneath the Item name in the Catalog. Click the drop down to select from the available options such as size, finish, and more to enable the Select button on the right of that line item.

See below example.

Step 1: Click the blue View item options button. Choose the finish.

Step 2: Click the blue Select button to add the Item to your project.