It is a representation of the cfLastMonth attribute. This attribute tracks the number of ladder games played over the last 30 days with a maximum value of 25.
The color, pure red at cfLastMonth=0, pure blue at cfLastMonth=25 and pure green in the middle, scales evenly with the numerical value over that range.
This is simply a quick and easy way to eyeball who is active on the ladder and who is not with more granularity than the red inactive we currently used on the main rosters or the old yellow inactive which was tracked by cfLastWeek.
It is kinda silly but it was easy to slap up there. If its useful, great. More than anything I intended it as a gag.