With the file extension ADO so-called Adobe Duotone Options files of the image editing software Photoshop are marked. An ADO file saves values of two colors for a duotone setting to the application on a grayscale image. In the German version of the mode is called duplex.
The duplex dialog box can be reached under 'Picture-> Mode-> Duplex'. In this box values can be saved and loaded. Usually ADO files are stored under Preferences / Duplex. This makes it easy to reuse useful settings and also use them for batch processing.