Customize Task Manager colors with Task Manager Modder
Task Manager Modder is just the tool for the job. Launch it, adjust the sliders (or enter hex codes) until the color scheme meets your particular needs, then apply the changes. You can also load a custom .BMP for the memory and CPU bars. To fall back to the Windows default, just click the Restore Task Manager button.
As with any tweaking tool, it's a good idea to create a system restore point before making any changes.