An ACO file is a color file used by Adobe Photoshop, a raster graphics editing program. It stores a color swatch, which is a collection of predefined colors, along with the name of each color, which is displayed when hovering the mouse over the color in the color palette. ACO files are used for creating images that conform to certain color standards.
ACO files are useful for controlling colors for different publishing media, such as digital image files and paper prints. Several color swatch files are included with the Adobe Photoshop installation in the /Presets/Color Swatches/ directory, which include colors for the ANPA, FOCOLTONE, HKS, PANTONE, TOYO, and TRUMATCH standards.