The word viridity describes a quality of being green, or a degree of how green something is. You could say that on a brilliant summer morning, the viridity of your front lawn is stunning.
Derived from a Latin root meaning "green," viridity is simply an old-fashioned or fancy way to say "greenness," literally or figuratively. In figurative use, viridity and greenness refer to naivety. One explanation of this comes from a green, unripe fruit that isn't yet mature or developed and applied to a person who is new at or unexperienced in something. You may admire the viridity of your lawn, but offer sympathy and guidance to your neighbors whose viridity in gardening currently has their lawn looking a little dry and brown.