Check this box if you want to view the photo in black and white (gray scale). This only changes the appearance of the photo on the Adjust Photo window. The original photo does not change. You may uncheck this box at any time to view the photo in color again.
Viewing the photo in black and white is useful if you have a black and white printer. As you adjust the photo, its appearance on the screen will more closely resemble how it will print.
Checking this box temporarily disables the Color Depth slider.