Potential
HD ratings improve until they reach their "cap" with practice time. Based on the color of a rating, here is how much that rating will improve before it "caps." Ratings with high potential develop faster, and ratings with low potential gain slower. A lot of players that starts with 30 WE won't even get ratings fully maxed (will come up a point or two short, since the last point or so is hard to get), since red/yellow ratings usually gain the most during the offseason.
Red: 0-3
Yellow: 3-7
Black: 7-21
Blue: 21-28
Green: 28+