I'm not an expert BUT... I suspect the diagonal line between the colors represents the duration of a stable orbit at that altitude. Anything with a more ellip

I'm not an expert BUT... I suspect the diagonal line between the colors represents the duration of a stable orbit at that altitude. Anything with a more elliptical orbit will drag when it's in the closer phase and decay. Anything with a lower orbit is already slowing to a tighter orbit (and more drag on the denser atmosphere).