This article describes two file formats that use the ACO file extension, including how to open both types.

What Is an ACO File?

A file with the ACO file extension is an Adobe Photoshop color file that stores a collection of colors.

Some ACO files may instead be project files used with the ArCon architectural software, but I don't have much information on them.

How to Open an ACO File

ACO files that are color files can be opened with Adobe Photoshop in a couple ways:

• The easiest method is through Edit > Presets > Preset Manager. Change Preset Type to Swatches and then choose Load to browse for the ACO file.
• Another way is to access the Window > Swatches menu. On the top right of the small window that opens (probably to the right of the program) is a button. Select it, choose Import Swatches or Load Swatches, and then select the ACO file.

While you can make your own custom swatches in Photoshop (through the Save Swatches option using the second method above), the program does include a handful of them when it's first installed. These are located in the \Presets\Color Swatches\ folder of the installation directory and are automatically loaded when the program opens.

The name of each color is also saved in this file, which you can see by hovering the mouse cursor over the color in the Swatches window in Photoshop.

The online image editor Photopea can import ACO files, too, through a similar swatches menu as Photoshop. Open that panel through Window > Swatches, and then select Load .ACO from the panel.

ACO project files are associated with software ArCon (planTEK).

How to Convert an ACO File

This is a special format used only in Photoshop, so there's no reason to convert one to any other format. In fact, Photoshop can't even see/browse/open the file if it's saved under a different file extension, so converting it would be useless.

Project files like those used by ArCon are normally saved in a proprietary format that's only useful within the program that created them. Plus, given that it's a project file, it likely holds other things pertinent to the project like images, textures, etc., so it's unlikely that it can be converted to some other format.

Still Can't Open It?

If your file doesn't open correctly with the programs described above, double-check the file extension to confirm that it really does read ".ACO" and not something that just looks similar. Some files share similar suffixes, even though they're not related and can't be opened in the same way.

ACF and AC3 files, for example, are strikingly similar but shouldn't be treated exactly like ACO files.

AC files are another example. They use a file extension that's just one letter off, but are actually unrelated to Photoshop and ArCon. Instead, AC files could be Autoconf scripts or AC3D 3D files.

Thanks for letting us know!