True yellow/green color separation for foliage is a difficult task without the proper tools. Well, there is a tool in Photoshop called Selective Color, and it is the most surgical approach to modifying color at our disposal. That's what I am going to show you today! The secret sauce of yellow-green color separation.
Elements does not have a Selective Color adjustment layer; this is a Photoshop-only feature. A possible workaround will be to use Elements+, an add-on created by Andrei Doubrovski.