You can't create new tags but you can recolour and rename the existing tags. You can also only choose from the colours the default tags have themselves (no custom colours, so you'd be left with duplicates if you did change any).
You can also delete the pre-made tags.
Yes, you can absolutely create new tags (as many as you want). Mavericks doesn't allow for new colors beyond the original eight (red through gray + white/non-colored). I hope Apple enables users to choose a full range of colors in the future; that would be a very nice feature.