Desktop Windowing Systems and Task Recognizability
One limitation of "windows" on modern computers is that they all look the same. Nearly all applications use the same coloring, layout, and shape of the overall window. While this consistency is good for recognizability and consistency in UI interaction, it's like having the papers on your desk all in a company's letterhead. It looks nice, but it doesn't promote at-a-glance recognition like yellow stickies and other notepaper can.
One way to distinguish windows (or applications) without sacrificing consistency is to highlight the window frame or client area background with a different color. This would change the requirement on the user to read window titles or the application layout (which requires having a substantial portion of the window not covered by other windows).