(The box with shades of blue, containing the avatar, on the very far left hand side)
Of course, the colours should only be for staff members. Although, using the Admin Control Panel, perhaps admins can chose certain hex codes (shade #1 and #2, for top and bottom) for the background colour as well as assign background colours for different user-groups. The colours which are used are totally up to the owner of the board and this feature should have the ability to be disabled.
The main advantage of this feature, is so that users can tell the difference between staff and regular members. One disadvantage is that board owner(s) may "over-use" this feature and assign a colour to all user groups, hence defeating the purpose of making the staff stand out.
Either way, I still stand with this idea.