What Is The Meaning Of Green And Greens In These Phrases?

GREEN is an adjective and refers to the color…. for example pepper green means the color of the pepper is green….
GREENS is a noun and refers to the plants.. specifically the leafy plants… so in mustard greens, it means that the leaves of the mustard..
SPRING ONIONS– A young onion before the bulb has enlarged; eaten in salads

green for beans and peppers describes a particular color of vegetable as disinguished from yellow beans and yellow or red peppers
With mustard greens, the greens here refer to the vegetalle itself as a distinct vegetable . Spring onion are usually your first green onion that is ready to be picked when there are young before they become bitey

Beans green just means green beans.
Mustard greens are a leafy vegetable that looks similar to green leaf lettuce, spinach, etc.
Pepper green is just a green bell pepper.
Spring onions are just green onions; the ones held together in bunches held together with the little rubber bands.

The green in beans just specifies the type of bean ie. green bean. The mustard greens refer to the leaves from the plant.