As you can see we had green bottle caps and white bottle caps for this sorting activity. I made 3 labels for the 3 section tray. I just used a green post-it note for the "Green" label, and a white index card cut down to size for the "bottle cap sorting" and "white" labels. My daughter was to young to actually read the labels at this point, but it really benefits kids to get used to being in a print rich environment from a very young age. As adults we're always dealing with labels and signs so it doesn't hurt to get them in the habit.
My daughter grasped the concept very quickly and had lots of fun sorting all the bottle caps.