This is one of over 4626 effects generated by Filter Forge, a Photoshop plugin that allows you to create your own filters.
To open this filter, you must have Filter Forge 1.0 or higher installed on your computer. If you don't have it installed yet, click here to download Filter Forge
This effect filter remaps the image's own color palette onto itelf. This may sound a bit strange but it results in very interesting color changes - ranging from solarization to two color images.
colors reduction solarization