Use ng-class.
ui.bootstrap has a good example of this for its rating control, using this technique:
ng-class="{'label-warning': percent<33, 'label-info': percent>=33 && percent<66, 'label-success': percent>=66}"
This works for progress bars as well. You just need to define percent somewhere.
http://plnkr.co/iBliMPYnWbsJJCtzqWTu
Update:
Here are classes intended for bootstrap progress bars:
ng-class="{'progress-bar-danger': percent<33, 'progress-bar-info': percent>=33 && percent<66, 'progress-bar-success': percent>=66}"