It seems to me that a player that is listed at a given useage, let's say 110 is in the green. let's say his number is actually 110.2, then he might be considered "ready," but since he is fractionally over, he shows up as yellow. I think it just reflects a fractional useage...and the color is a guide that he's about ready to bump up to a higher number.
I've seen the opposite too...my player is 110 and in the yellow, he gets a day off, and after the game is now 110 in the green.
So not much difference. If he's ready...he seems to be ok to play without any detriment...whether his color is green or yellow.