It should be possible for notebookbar developers to define * a tab color in tabbed notebookbars or * a background color of a (contextual) group of icons in the grouped notebookbar. The attached screenshot shows how MSO Word 2013 colors the contextual tabs to give them a visual emphasis so that the user can see that there is more to select then before. For a grouped notebookbar without tabs it should be possible to give the contextual group a own background color in a (pre-defined?) transparency level to not to be too offensive.
Created attachment 132532 [details] Screenshot of MSO 2013 ribbons with colored image and table tabs (at the right side)
As the notebookbar, just like all the UI in LO, is themed by the OS theme, it would be difficult to define a specific color for it, but defining a special OS themed color would likely be possible. *** This bug has been marked as a duplicate of bug 107128 ***
I don't know if Andreas really mean a similar thing than I do. Shades of gray (or whatelse is possible as system color) isn't shiny enough to get user's attention as desired with this bug.