In Photoshop you can easily create an image in one color mode and convert it to another, perhaps to get it ready for a specific print job.

You can change an image from its original mode (source mode) to a different mode (target mode). When you choose a different color mode for an image, you permanently change the color values in the image. For example, when you convert an RGB image to CMYK mode, RGB color values outside the CMYK gamut (defined by the CMYK working space setting in the Color Settings dialog box) are adjusted to fall within gamut. As a result, some image data may be lost and can’t be recovered if you convert the image from CMYK back to RGB.

Before converting images, it’s best to do the following:

• Do as much editing as possible in the original image mode (usually RGB for images from most scanners or digital cameras, or CMYK for images from traditional drum scanners or imported from a Scitex system).

• Save a backup copy before converting. Be sure to save a copy of your image that includes all layers so that you can edit the original version of the image after the conversion.

• Flatten the file before converting it. The interaction of colors between layer blending modes changes when the mode changes.

Convert an image to Bitmap mode

Converting an image to Bitmap mode reduces the image to two colors, greatly simplifying the color information in the image and reducing its file size.

When converting a color image to Bitmap mode, first convert it to Grayscale mode. This removes the hue and saturation information from the pixels and leaves just the brightness values. However, because only a few editing options are available for Bitmap mode images, it’s usually best to edit the image in Grayscale mode and then convert it to Bitmap mode.

Convert a color photo to Grayscale mode

Convert a Bitmap mode image to Grayscale mode

You can convert a Bitmap mode image to Grayscale mode in order to edit it. Keep in mind that a Bitmap mode image edited in Grayscale mode may not look the same when you convert it back to Bitmap mode. For example, suppose a pixel that is black in Bitmap mode is edited to a shade of gray in Grayscale mode. When the image is converted back to Bitmap mode, that pixel is rendered as white if its gray value is above the middle gray value of 128.

Convert a grayscale or RGB image to indexed color

Converting to indexed color reduces the number of colors in the image to at most 256—the standard number of colors supported by the GIF and PNG‑8 formats and many multimedia applications. This conversion reduces file size by deleting color information from the image.

To convert to indexed color, you must start with an image that is 8 bits per channel and in either Grayscale or RGB mode.

Conversion options for indexed-color images

When converting an RGB image to indexed color, you can specify a number of conversion options in the Indexed Color dialog box.

A number of palette types are available for converting an image to indexed color. For the Perceptual, Selective, and Adaptive options, you can choose using a local palette based on the current image’s colors. These are the available palette types:

Creates a palette using the exact colors appearing in the RGB image—an option available only if the image uses 256 or fewer colors. Because the image’s palette contains all colors in the image, there is no dithering.

Uses the Mac OS default 8‑bit palette, which is based on a uniform sampling of RGB colors.

Uses the Windows system’s default 8‑bit palette, which is based on a uniform sampling of RGB colors.

Uses the 216-color palette that web browsers, regardless of platform, use to display images on a monitor limited to 256 colors. This palette is a subset of the Mac OS 8‑bit palette. Use this option to avoid browser dither when viewing images on a monitor display limited to 256 colors.

Creates a palette by uniformly sampling colors from the RGB color cube. For example, if Photoshop takes six evenly-spaced color levels each of red, green, and blue, the combination produces a uniform palette of 216 colors (6 cubed = 6 x 6 x 6 = 216). The total number of colors displayed in an image corresponds to the nearest perfect cube (8, 27, 64, 125, or 216) that is less than the value in the Colors text box.

Creates a custom palette by giving priority to colors for which the human eye has greater sensitivity.

Creates a color table similar to the Perceptual color table, but favoring broad areas of color and the preservation of web colors. This option usually produces images with the greatest color integrity.

Creates a palette by sampling the colors from the spectrum appearing most commonly in the image. For example, an RGB image with only the colors green and blue produces a palette made primarily of greens and blues. Most images concentrate colors in particular areas of the spectrum. To control a palette more precisely, first select a part of the image containing the colors you want to emphasize. Photoshop weights the conversion toward these colors.

Creates a custom palette by giving priority to colors for which the human eye has greater sensitivity. Applies when you have multiple documents open; takes all open documents into account.

Creates a color table similar to the Perceptual color table, but favoring broad areas of color and the preservation of web colors. This option usually produces images with the greatest color integrity. Applies when you have multiple documents open; takes all open documents into account.

Creates a palette by sampling the colors from the spectrum appearing most commonly in the image. For example, an RGB image with only the colors green and blue produces a palette made primarily of greens and blues. Most images concentrate colors in particular areas of the spectrum. To control a palette more precisely, first select a part of the image containing the colors you want to emphasize. Photoshop weights the conversion toward these colors. Applies when you have multiple documents open; takes all open documents into account.

Creates a custom palette using the Color Table dialog box. Either edit the color table and save it for later use or click Load to load a previously created color table. This option also displays the current Adaptive palette, which is useful for previewing the colors most often used in the image.

Uses the custom palette from the previous conversion, making it easy to convert several images with the same custom palette.

For the Uniform, Perceptual, Selective, or Adaptive palette, you can specify the exact number of colors to be displayed (up to 256) by entering a value for Colors. The Colors text box controls only how the indexed color table is created. Adobe Photoshop still treats the image as an 8‑bit, 256‑color image.

To specify colors to be included in the indexed color table or to specify transparency in the image, choose from the following options:

Provides options to force the inclusion of certain colors in the color table. Black And White adds a pure black and a pure white to the color table; Primaries adds red, green, blue, cyan, magenta, yellow, black, and white; Web adds the 216 web‑safe colors; and Custom lets you define custom colors to add.

Specifies whether to preserve transparent areas of the image during conversion. Selecting this option adds a special index entry in the color table for transparent colors. Deselecting this option fills transparent areas with the matte color, or with white if no matte color is chosen.

Specifies the background color used to fill anti-aliased edges that lie adjacent to transparent areas of the image. When Transparency is selected, the matte is applied to edge areas to help blend the edges with a web background of the same color. When Transparency is deselected, the matte is applied to transparent areas. Choosing None for the matte creates hard-edged transparency if Transparency is selected; otherwise, all transparent areas are filled with 100% white. The image must have transparency for the Matte options to be available.

Unless you’re using the Exact color table option, the color table may not contain all the colors used in the image. To simulate colors not in the color table, you can dither the colors. Dithering mixes the pixels of the available colors to simulate the missing colors. Choose a dither option from the menu, and enter a percentage value for the dither amount. A higher amount dithers more colors but may increase file size. You can choose from the following dither options:

Does not dither colors but instead uses the color closest to the missing color. This tends to result in sharp transitions between shades of color in the image, creating a posterized effect.

Uses an error-diffusion method that produces a less-structured dither than the Pattern option. To protect colors in the image that contain entries in the color table from being dithered, select Preserve Exact Colors. This is useful for preserving fine lines and text for web images.

Uses a halftone-like square pattern to simulate any colors not in the color table.

Helps to reduce seam patterns along the edges of image slices. Choose this option if you plan to slice the image for placement in an HTML table.