Set the maximum number of colors in the image’s palette. Reducing the number of colors in an image can help to reduce file size. Other transformations can also affect color, such as Grayscale and Hue/Saturation/Lightness. If you are using more than one, consider the order to apply them for the desired results.
If you add the IMQuery transformation to a policy, use the im variable, and select Max Colors, you can use a query string to adjust the colors of any image using this policy. See Syntax and Examples for the syntax for the query string parameter.