The first stamp I'd like to share with you I don't own myself. Thanks to the internet I found an image and I am happy to show it today, because it is such a matching stamp!
Today not only is a 'green' themed Sunday Stamps day, but today also is the start of the Persian New Year: Now ruz or Norooz.
For this celebration, people arrange 'haft seen', a tabletop with 7 items which start with the letter 's'. One of the items is Sabzi, which literally means both 'green' and 'vegetable'.
This stamp shows Sabzi: the green sprouts of, for example, wheat or lentils, bound together by a red ribbon.

(I think the tulip is optional, I have never seen it in real/on pictures so far.)
For anyone who is celebrating this New Year: Happy Norooz!
I happened to have joined a chaincard project themed 'green' via Instagram recently.
The postcard started its journey in the Netherlands and after a short stop in Taiwan (ROC), Japan and Italy it returned with these stamps containing the colour green:

Austria issued this stamp for the 20th anniversary of the Alps Adriatic Philately:

Much green you can see in this stamp - one of my favourites - from Japan:

'Green' of course also has to do with environment friendly issues.
These environment friendly themed stamps are from Spain:

The following five are from a 'green' ('groen', environment friendly) ten stamp sheet from the Netherlands:

'Het nieuwe werken, groene winst' means 'the new way of working, green (environmental) profit'. The laptop represents working-at-home, in order to decrease car use and traffic.
Some of the other stamps, of which the one top right says 'groene post, een goed bericht', which mean 'green mail, a good (positive) message':

More about this stamp serie (in Dutch, but I think the non-Dutch speakers will enjoy the pictures, too) on this site and about the design process by Gesina Roters on this site.
Other 'Green' stamps (among them the 'denk groen', 'think green', stamp sheet you can find on this site.
See more stamps on the theme 'Green' on the Sunday Stamps blog and the links under the blog post.