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?

Call sidebar from a template

I need to call a sidebar into my header. I’ve switched to the new twenty twelve theme and created my own child theme. With this new theme I decided to keep things organised and together, so I put my widgets functions, stylesheet and two new sidebar templates (one for header and one for footer widgets) in one folder called pgwidgets.