Traditional Photographic Techniques
While Photoshop is terrific for spicing up image color, it also offers a few tricks for spicing it down. However, if you use just the Desaturate tool (Image>Adjustments>Desaturate), the best you’ll get is a basic grayscale image, as there are no degrees of adjustment with that tool. Useful, but not terribly versatile.
Working the Saturation slider at Image>Adjustments>Hue/Saturation to the left will gradually desaturate the image until all that is left is the same basic grayscale image.
You can also desaturate any selected color. This is especially useful if you’d like or need ...