An ACB file is a custom colors file used by Photoshop, an image-editing program. It contains color records, which include color names, color codes, RGB and CMYK components, lightness, and chrominance. ACB files also contain header information for the book version, title, description, color count, and other file information.

Color books are used for creating images that conform to certain color standards. For example, some color catalogs are better for screen displays while others are better for scanning or print. In Photoshop CS5, you can choose a color book when selecting the Color Libraries button from the Color Picker window. Several ACB files are provided with the Photoshop installation within the \Presets\Color Books\ directory. Photoshop provides color books for color catalogs such as ANPA, DIC, FOCOLTONE, HKS, PANTONE, TOYO, and TRUMATCH. NOTE: Adobe InDesign and Illustrator also use color books stored in ACB files.