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

Keep in mind that the Desaturate command permanently alters the original image information in the background layer. This command has the same effect as setting Saturation to minus (‑)100 in the Hue/Saturation adjustment. For nondestructive editing, use a Hue/Saturation adjustment layer.

Invert colors

The Invert adjustment inverts the colors in an image. You can use Invert as part of the process of making an edge mask to apply sharpening and other adjustments to selected areas of an image.

When you invert an image, the brightness value of each pixel in the channels is converted to the inverse value on the 256‑step color-values scale. For example, a pixel in a positive image with a value of 255 is changed to 0, and a pixel with a value of 5 is changed to 250.

Do one of the following:

• Click the Invert icon  in the Adjustments panel.
• Choose Layer > New Adjustment Layer > Invert. Click OK in the New Layer dialog box.

Create a two-valued black and white image

The Threshold adjustment converts grayscale or color images to high-contrast, black-and-white images. You can specify a certain level as a threshold. All pixels lighter than the threshold are converted to white; all pixels darker are converted to black.

Posterize an image

The Posterize adjustment lets you specify the number of tonal levels (or brightness values) for each channel in an image and then maps pixels to the closest matching level. For example, choosing two tonal levels in an RGB image gives six colors: two for red, two for green, and two for blue.

This adjustment is useful for creating special effects, such as large, flat areas in a photograph. Its effects are most evident when you reduce the number of gray levels in a grayscale image, but it also produces interesting effects in color images.

Apply a gradient map to an image

The Gradient Map adjustment maps the equivalent grayscale range of an image to the colors of a specified gradient fill. If you specify a two‑color gradient fill, for example, shadows in the image are mapped to one of the endpoint colors of the gradient fill, highlights are mapped to the other endpoint color, and midtones are mapped to the gradations in between.