Idioms are odd. They say what we mean them to say. But they are made of words and ideas that sometimes appear to have little to do with that meaning.
Vita has ``green fingers'' (or if she is American, ``a green thumb''). Sykes is ``a rough diamond.'' Sidney persistently ``talks the hind leg off a donkey.'' We could say Vita has a skillful way with plants, that Sykes has character but no finesse, and that Sidney chatters ceaselessly. One of the incalculable assets of a rich language is when it offers a variety of ways to say much the same thing. An ingenious choice of word or phrase from such a potpourri may result in just a little more color, a touch more humor - or it may discriminatingly adjust a sensitive balance.
Where do idioms come from?
Some have origins so obscure that nobody seems quite certain. Others are quite obvious. Still others have theories attached to them that tend to be repeated rather than ever really explained. Attempting to trace their first meanings can easily become a sort of game.
Pursuing words and phrases with an eye to their background and associations is not unlike such elusive pastimes as trying to catch moonbeams, darting from pillar to post, or going on a wild goose chase.
``Green,'' however, seems easy enough. Greenness is associated with the fresh, the new, with springtime. It is the color of plant growth. So if your African violets (unlike most people's) flourish on your windowsill rather than wither, why shouldn't your fingers and thumbs be designated ``green''?
A diamond in its uncut and unpolished state is ``rough.'' So a good-hearted person might be outwardly gruff or coarse, but could with a bit of work be just as good on the surface.
As for talking the hind leg off a donkey.... Who knows? My theory is that donkeys, who seem to stand forever on beaches waiting to give rides to children, have a patience quite beyond the attainment of any other creature. So for a talker to talk the hind leg off a donkey would be to try a listener's patience beyond all endurance.
But perhaps you have a better theory?