Limit widget to certain sidebar?

I have multiple “sidebars”, but not all of them are the same size. Not all widgets fit in all sidebars (for example, I have a “footer sidebar” where the client can place custom widgets, but they are wide, and don’t fit at all in the “real” sidebar). I want to give an indication when a widget is placed in a sidebar where it would not belong (change the header text color, for example). What would be the best way to do this?