You have to do it in skinstudio. On the left hand side of SKS -- Backgrounds/Edit explorer background bitmaps/explorer backgrounds Edit the very top one "active window. You can use a 10x10 px, 100x100 px if you wish to see it better when making it. This will change the left hand side.
I think you really only have to do the right hand side for what you are looking for. Open skinstudio. Edit Explorer & IE Shell Parts open that and click on Edit Vista & 7 Explorer Parts then go to Folders Bar. Look for the tab Extra settings tab and and click on it you will see Bacjground color(win7) On the right you will see a color drop down menu. Open it and pick you color.
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