yeah, there is a general rule. low = 0-5 points, medium = 6-20, high = 21+. you can send evals to break high potential (and low, technically) into 2 categories - high/high and low/high. low/high is 21-27 points of improvement, and high/high is 28+, you can usually tell from the eval its high/high if it has things like MASSIVE or *tremendous* improvement mentioned. i had a list of all the high/high messages vs low/high, and dacj published it somewhere convenient. so maybe he will come along and post it, i don't know where to find it anymore.
im not sure where you would have seen blue numbers give 10-15, are you sure they capped out? low work ethic guys won't hit their caps. keep in mind potential just signifies what the maximum is, not how fast they get there (that is a combination of potential, work ethic, practice time, playing time). i dont think red numbers can go up by more than 5, or maybe more than 6. the player will turn red after hes gained some points off a black (medium) number though, so are you sure that player in question didn't start black, and end red?