Updated.... The color codes have adjusted so the population is more accurately displayed.
Blue= Not Open
Green= Very High Activity (New worlds)
Light Green= High Activity (New worlds that are still growing or worlds that are starting to lose players)
Yellow= Moderate Activity (Worlds that are losing players and starting to approach the world wonder era)
Orange= Low Activity (Worlds that are really close to or already in the world wonder era)
Red= Very Low Activity (Worlds that are in the world wonder era or that have been won)
Bold= Closed