

Double_Helix_DNA : the color is computed based on the total number of players shown in ladder (you can check at
https://zero-k.info/Ladders/Full , 1466 as I wrote the post). For some reason, each time the server is reset (I mean the software is restarted, or the server physically) the number of active players will decrease swiftly (as far as I got because the daily computation of active player differs from the start computation of active player).
To summarize, if you are in the lower part of blue ranks, and server is restarted and you loose once, you will probably change color.
Once more players will be considered active, which usually takes a couple of weeks in my experience and if you play exactly at same rating you will go back again to blue.