Ah, in this case the most obvious way to do it is to use 'Edit > Colors > Convert to scalar fields' (you can only choose to export the Green components, but you could also use the other components to pre-select points with low blue and red values).
Then use the scalar fields to filter the point. You can do it interactively (with the scalar field display properties section in the cloud properties once highlighted). Play with the min and max displayed values (there's also an option to hide the points falling outside of this range in the second pannel).
Eventually, you can use 'Edit > Scalar fields > Filter by value' to extract the corresponding points as a new cloud.
And there's another option: the Colorimetric segmenter plugin (
https://www.cloudcompare.org/doc/wiki/i ... r_(plugin)). Sadly there's no real documentation yet for this plugin, but you can try to play with it, it's pretty straightforward if I remember well.