Green indicates to some people that their grades are good and they are on track when this may not be the case. Blue is a relatively neutral colour across cultures (there's a reason it's used by Facebook, Twitter etc) and so it makes more sense for the numeric grade (performance) bar to be blue instead of green. 

We did consider banding the bar into different colours to indicate poor, average and good performance since different departments/programmes/modules have differing fail and pass grades it would have caused confusion, especially for those studying across departments. Blue is the safest bet.