If a player loses rating points during the season, there's going to be a chance he changes colors. If low potential/red is up to 7 points, then if he starts the season with a 10 rebounding and his cap is 17, he's red because he's 7 from his cap. When he loses a point and drops to a 9 in rebounding, he's now 8 away from the cap...that's medium potential, so the color will change to black to reflect that. The same will happen if you go from normal to high potential distance from the cap due to lost points and the color code will change from black to blue accordingly.
In essence, thanks to the color change, you know exactly where the kid's cap in that skill is going to be.