You could have opened the Preview window, which allows for Tagging, Star rating and adding a Color label. By default the number keys will add a Color label and Alt-# keys will add a Star rating. The t shortcut key will toggle the Tag checkbox on and off.
If you started with the Preview window opened, then the i shortcut key will open the Metadata (IPTC) Info dialog. You can apply a Color label, Star rating or Tag the image in this dialog, using the widgets below the thumbnail.
You might have opened the Image menu which has options to add a Color label, Star rating or a Tag. I hope this helps.
--Bob
