Viridity can mean green, as in color or youth. For example, you can praise the viridity of the lawn, or the viridity of a young lady.
Viridity is a noun, which usually means it is a person, place, or thing. In this case, it is an idea.
In the spring, I am inspired by the viridity of the grass and trees.
In this sentence, viridity is the object of the preposition, a noun.
It has a Middle English origin, based on the Latin word viriditās, which means green.