A greyscale image looks like a traditional black-and-white photograph. Use the Greyscale command to convert a colored image into a greyscale image with a palette that contains white, black, and 254 shades of grey. Each color in the image is replaced with a grey that matches its lightness value.
Choose Image > Greyscale. Be aware that the Greyscale command changes the color depth of the image to 8-bit.
Here are some methods of creating a greyscale image without reducing the bit depth to 8.
Use the Channel Mixer command (Adjust > Color Balance > Channel Mixer) and mark the Monochrome check box. This command allows you to adjust how much of the image's red, green, and blue channels are used to create the monochrome image.
Use the Colorize command (Adjust > Hue and Saturation > Colorize) and set the Saturation value to 0.
Use the Hue/Saturation/Lightness command (Adjust > Hue and Saturation > Hue/Saturation/Lightness) and set the Saturation value to -100.
Use the Hue Map command (Adjust > Hue and Saturation > Hue Map) and set the Saturation shift value to -100.
Note that the Colorize and Hue Map methods will produce the same visual effect as the Greyscale command. The Channel Mixer and Hue/Saturation/Lightness methods use different conversion algorithms and will produce a slightly different but quite similar visual effect.
Creating Black and White Images
Reducing the Number of Lightness Levels