It is accepted that the bulk of the "green stones" that are passed are made up of olive oil that has congealed. The truth is that the olive oil will cause greater than normal bile to be excreted and this will flush out some stones with it. These smaller stones become cores or nuclei for the green material to aggregate around.
Additionally, if the larger stones are loose enough, they will wash out too and will be noticed as stones that are often not green.
The problem is that some "anti-flushing experts" will use the green stones to claim that flushing does not work. Sadly, they are missing the real stones which often come out after several flushes.
Flushing does work, but not always on the first or second try.