There is a tool called <em>Aero Adjuster</em> that works as a tray-icon in the taskbar and changes the colors of your Windows 7 Aero theme every time you change your wallpaper. Furthermore you also have the option to change brightness and/or to invert the colors, ignore black, white and grey tones and to only choose a specific region of the wallpaper (e.g. Top right, Bottom left).
It works automatically but can also be applied manually after changing options.
Download Aero Adjuster here: http://samusaranfreak.deviantart.com/art/Aero-Adjuster-261383189