The hieroglyph refers to the hills, and mountains, on both sides of the Nile River, and thus the green references the verdant black farming land adjacent to the river proper.
It is a determinative hieroglyph, simply conveying a meaning, and has no phonetic value.
[1] Various colors, and patterning,[2] may adorn the rest of the hieroglyph when the bottom is green.
For example in the Merneptah Stele, foreign lands are mentioned, including the name of Hatti.
[8][9] The nine foreign lands used for the Nine Bows are also iconographically shown inside of cartouches, with their names.