This is the 857th most frequent Japanese word.
緑
Green.
Here, '緑' modifies '葉' as its adjective form, meaning 'green'.
緑の葉が風に揺れている。
The green leaves are swaying in the wind.
In this sentence, '緑' is used as a noun to mean 'greenery' or 'verdure' in general.
日本の田舎には美しい緑が広がっている。
In the Japanese countryside, beautiful greenery spreads.
Here, '緑' is referred to as a specific color, named as 'green', and is treated as a noun describing the concept of green.