This effect was generated in Filter Forge, a Photoshop plugin that allows you to create your own filters.
This is a simple colour processing filter that takes the shadows, midtones, and highlights of your image and assigns them to one of four colours. At low "Colour Smoothness" values, the results can look like poster art using process colours (e.g. Pantone, Toyo, ANPA). Higher values can approximate the look of a more sophisticated quadtone or produce effects that simulate cyanotype or sepia.
And, of course, you can use fewer than four colours by assigning the same colour to more than one input, which can also create some unique effects – just don’t use the same colour for all four inputs! ^_
Have fun!
posterization quadtone process_colour scarykitty cyanotype sepia pantone