Using the Desaturate command

The Desaturate command converts a color image to a grayscale image in the same color mode. For example, it assigns equal red, green, and blue values to each pixel in an RGB image to make it appear grayscale. The lightness value of each pixel does not change.

This command has the same effect as setting Saturation to -100 in the Hue/Saturation dialog box.

Note: If you are working with a multilayer image, Desaturate converts the selected layer only.

To use the Desaturate command:

Choose Image > Adjustments > Desaturate.