Today's Visual Studio Extension is one you'll love if you constantly keep multiple instances of Visual Studio running (like I do). It's simple, yet once you see it, brilliant in hindsight (and it's open source too).
Adds a toolbar that allows to set the color of the title bar on a per sln file basis. Chosen color is saved for every sln path and will be set on startup.
The left button pops up a color picker, the right one resets the setting back to the Visual Studio default and removes the settings entry for the current sln file (if any).
(Text color is automatically chosen between white or black)