What do the different colors of weather stations indicate on SkyVector? In this particular map



from SkyVector, what are the meanings of the different colored circles, and for the different amounts of 'fill-in'?  


It's obvious by hovering over them, that they are weather stations, as the weather info will pop-up, but there doesn't seem to be anything (to my untrained eye) about the weather pop up that would indicate a need for the different colors.
It's also obvious that the color doesn't indicate something permanent about the station, as last Friday, there were large swathes of different colors across the USA, while today almost everything is green.
Colors
At present1, they mean (image source):

For more details, please see this description.
Fill-ins
These are called sky/cloud coverage.

Image Source

1 From SkyVector:

The color is controversial because the FAA is making the NWS swap the
colors for VFR and MVFR. When the new aviationweather.gov becomes the
current aviationweather.gov, we'll change our colors to match.