An ACT - Adobe Color Table File is a Color Look Up Table (CLUT) type file. These files are used in Adobe Photoshop to store a table of predefined colors, for use when the user is in the "Save to Web" dialog window, the CLUT file is applied. To use the color tables, whenin the "Save to Web" dialog, select the Color Table menu and choose "Load Color Table".
The colors in the CLUT file can be edited to help reduce file size for web page use and you can view the effects of the color reduction "on the fly". This allows you to decide if the image quality is damaged and maximize the color to associated file size ratio before the image is actually uploaded to the web site. This allows you to save server space, minimize page load times and still maintain image quality.
Note: CLUT files are not used by .JPG and .PNG files.