locked The background colors for call signs

Bernd - KB7AK

Newbie user here, I have been trying to understand what the colors mean. Except for stations that I have worked before (that are on gray background and have the "B4" added to the call sign), all other call signs are on the same color, it seems to be the color for continent. Once I have worked a continent, why would still all other call signs from this continent show the same background? 

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