Check the "Which are labelled" box to copy files which have a particular label. When this box is checked, only the files which have any of the checked labels will be copied.

Keep in mind that if you use Synchronize! on more than one Mac, the labels on the two Macs may be different. The color of the label doesn't change, but the name of the label may. It's best to keep the labels on your Macs, and the order of the labels in the Finder's menu, the same to prevent confusion.