“One of [English landscape painter] Constable’s ‘secrets,’ not lost on Delacroix and other artists, was his method of creating rich, vibrant greens in foliage and grass… by dabs and strokes of several greens. …
“The variations produce scintillation and ‘depth’ because of a certain amount of fusion in the eye of the observer.”
(Source: Calvin Harlan, Vision and Invention: A Course in Art Fundamentals, page 107
Or in Delacroix’s own words, the secret of Constable’s green:
“… lies in the fact that it is composed of a multitude of different greens. The lack of life and intensity in the greenery of the common landscape painters is caused by the fact they usually paint it in a uniform green.”
(Source: Delacroix’s Journal I, 5 March 1847, p281, quoted in Art in Theory 1815-1900, edited by Harrison, Wood, and Gaiger, p980).
Think of hatching for a pencil drawing, where you use tiny lines to build up an area, rather than a solid line or area of blended tone. To do it in paint, use various hues and tones of green overlapping and layered, with specks of what’s below showing through, rather than one ‘perfect green’ only.
If you zoom in on one of the photos of Constable’s paintings on the London National Gallery website, for instance Stratford Mill, you can see how much variation there is in a small area (and not only in the greens!).