Okay, because I'm not making it up as I go along
...
There are two types ... yellow stars are related to the number of posts a person has made, other colour stars are for "groups" and are not related to the number of posts a person has. If you are the member of a group (phanatics, classroom) your yellow stars don't show but your "group" star(s) do.
Number of yellow stars - related to the number of posts you have made
Number of other than yellow stars - related to the length of time you have been a contributing member of a particular group. Phanatics have been active for most of the last twelve months so they have two stars as this is their year two. Green was just a random colour choice for the phanatics. This is only year one of the Classroom group, so only one star there. I have five starts cuz its my Sandbox
Does that help?