Tetris 99 announced for Switch (aka tetris Battle Royal)! Free for Nintendo Online Members! Available Now!

[Yerm]

So it would seem the color icons are purely dependent on what level you are. for anyone that is curious -

01 - 09: Green
10 - 19: Light Blue
20 - 29: Dark Blue
30 - 39: Purple
40 - 49: Red
50 - 59: ???