During recording, clicking an element for the first time shows a green border, but clicking it again shows a yellow border. What does this mean?

While you are recording, Alchemy is capturing the objects you encounter into the "Site" entity.  Each object is stored in the "Site" as a set of intelligently selected locators.  When you encounter, or interact with, an object that is not already in the "Site", the highlight will be green.  That means Alchemy is storing a new object with a set of locators for tat object.  When you encounter an object during recording that is already existing in the "Site", then Alchemy will indicate that by highlighting the object in Yellow.