What is the ACO file format?
An .aco file is an Adobe Photoshop Color Swatch - a binary palette format used by Adobe Photoshop to store named or anonymous collections of colors for use in design and image editing workflows. Swatches appear in Photoshop's Swatches panel and can be loaded, saved, and shared between users and projects.
The format exists in two versions. Version 1 stores colors anonymously as a simple count followed by fixed-size color blocks. Version 2, introduced with Photoshop 7.0 (2002), appends a second block in which each entry carries a UTF-16 Unicode name, enabling meaningful labels such as Brand Red or Pantone 485 C. A Version 2 .aco file embeds both blocks, maintaining backward compatibility with older software.
Each color is encoded as four 16-bit channel values stored big-endian, giving 65,535 levels of precision per channel. The format supports RGB, HSB, CMYK, Lab, Grayscale, Wide Gamut RGB, and spot-color libraries including Pantone, Toyo, DIC, and FOCOLTONE.
GIMP and Krita can also read .aco files. For cross-application swatch sharing, Adobe later introduced the .ASE (Adobe Swatch Exchange) format; .aco remains Photoshop-native and is fully supported in current versions.
Security & safety
RISK: LOWBinary color palette data only; no executable content. Safe to open.
Format details
in a nutshellPrograms that open ACO files
Windows3 apps
Adobe Photoshop
Paid Swatches panel → hamburger menu → Load Swatches → select the .aco file. Colors appear in the Swatches panel. 
GIMP
Open-source Windows → Dockable Dialogs → Palettes → Import Palette → select .aco file. Free Photoshop alternative. 
macOS2 apps
Technical details
deep spec| Internal format | Proprietary flat binary (no container wrapper) |
| Magic bytes | 00 01 - first 2 bytes are the version number (0x0001 = Version 1 header) |
| Byte order | Big-endian (all values) |
| Format versions | V1 (anonymous colors); V2 (named colors, introduced with Photoshop 7.0, 2002) |
| Color precision | 16 bits per channel (values 0-65,535) |
| Supported color spaces | RGB, HSB, CMYK, Lab, Grayscale, Wide Gamut RGB, Pantone, Toyo, DIC, FOCOLTONE |
| Max colors | 65,535 (2-byte count field) |
| V2 color names | Stored as 4-byte length prefix followed by UTF-16 big-endian string |
| V2 backward compatibility | V2 files embed a complete V1 block before the V2 block for software compatibility |
| Typical file size | 100 B - 100 KB |
| Supported OS | Windows, macOS |
| Cross-app successor | ASE (Adobe Swatch Exchange, .ase) for sharing swatches across multiple Adobe applications |
| Released | circa 1990 (Adobe Photoshop 1.x era) |
| Latest version | Version 2 (introduced with Photoshop 7, 2002) - current |
| Specification | www.nomodes.com |
ACO conversions
Community Q&A
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