I have tried to make sense of offseason improvements before, but there doesn't seem to be any true way to predict it. Work ethic definitely plays a role, and practice plans definitely do not. Areas with blue potential tend to grow the most, but other than that I think it's a crapshoot. For example, this offseason I had a PG with high potential in both passing and ball handling improve 8 in one category and 2 in the other. The category with the growth of 8 turned black while the other category stayed blue, so the category with less growth actually had more potential to grow, but grew less.
There some to be some hard rules to offseason growth, but the majority of it is random.