I need a reference that lists the greenscale color of leaves (or plants).
It would be helpful to have more information about what you are trying to do. There are charts which show different shades of green in leaves as evidence of nitrogen levels, but these are mainly used in agricultural settings, such as rice cultivation.
If you are looking for charts used in horticulture, one tool for answering questions about plant colors is the Colour Chart put out by the Royal Horticultural Society. This is a set of numbered cards, in every imaginable color, that you can hold up to a flower or leaf in order to determine the standardized color. There is an extensive set of colors in the green scale. The problem with the RHS color chart is that it is expensive. If you are able to visit the Miller Library, we have a copy of this color chart that you are able to use in the library.
The Azalea Society of America has a page of information on color systems which includes the RHS Colour Chart, the Universal Color Language, Munsell, and others.