It would be good if a user could assign specific background and foreground colours to the diagram elements - for example, OLTP entities would be in blue while OLAP entities would be in green.
Making the style assignment free form allows the designer to decide how/why/which colours to use and have it match their own internal documentation.
In this way it is easier to distinguish at a glance, specific functionality/design needs within a single or group of diagrams.
If you could have a user definable list of styles that a user can assign to a diagram element would make this a very useful feature.