Posterizing With Index Color
Photoshop CS6 Tutorial
How to Posterize with Index Color in Photoshop
To posterize an image, start with a continuous tone photograph. The process converts your image into a simplified image with limited shapes and colors.
Open a photograph in Photoshop (File > Open).
Duplicate the image (Image > Duplicate) and close the original.
Choose Image > Mode > Indexed Color....
Make sure the Preview checkbox is selected. Set the Palette to Local (Selective) and the Forced to Black and White. Dither should be set to None.
Start with the Colors set to a low value. Increase the number one value at a time and watch the preview. You can also use your Up and Down Arrow Keys to step through the choices.
Once you are satisfied, click OK. Convert the image to RGB by choosing Image > Mode > RGB Color.