The difference between the colors is-
green symbolizes the world has been protected by BattlEye since the creation of the world
yellow symbolizes the world has been protected with BattlEye since a certain date after the worlds creation.
When you look at the world list on Tibia's website and you hover over the BattlEye symbols it will remind you what it means. Also, to know when a world is created click on the world on the world list and look where it says creation date.
Also if you plan to world transfer just a heads up-
"Characters can only move to a game world which has been initially protected by BattlEye if they also come from a game world which has been protected by BattlEye right from the start."
Sources-
https://www.tibia.com/news/?subtopic=newsarchive&id=4264&fbegind=4&fbeginm=6&fbeginy=2004&fendd=4&fendm=7&fendy=2020&flist=11111111
https://www.tibia.com/gameguides/?subtopic=manual§ion=products#changeworld
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