Background color for Modal

You are right. Setting colors for many objects on diagram is not comfortable. I think we can improve this by allowing users to change format properties for all selected objects. We will put it on our road map of UI/UX enhancements. 

You can also use subject areas feature to surround particular tables. Subject area can have background itself, so you can set background color not for the whole model but a part of it.

What about something as simple as setting the background of the entire model. This would be easiest to maintain as it is only one setting.

Subject areas and possibility of changing colors of diagrams elements are the only elements designated to manage visual side of the model.

The color of model area is a part of the whole Vertabelo visual design and it has to correspond to others elements - panels, controls, grid etc.. So we don't plan to add user possibility of changing it (however it is very simple from implementation point of view).