FOSS gamedev and creative worlds

I found in the 0.4.12 that the client has now colours in the serverlist and on the numbers that indicate players on it. Also, I found that I don't know what this means. Can somebody explain what does the colours mean? So far I've found:
-Gray (Players and/or server name)
-White (Players and/or server name)
-Green (Players and/or server names)
-Yellow (Players)

Red = full
Yellow = close to full
Green = lots of slots remain available
White = empty
Gray = ?

Thanks Calinou ^^
Also, do you know the difference between the names of the servers being white or gray?

If the name of the server is grey, then that means that it's running a higher version of minetest than your client. (i.e. it is using a minetest version above yours, or a dev version of minetest.). I believe that it will remain that way no matter how many players are on it.

Thank you Amaz. Very useful, I guess I will have to start using dev versions.