This technique reduces the color palette of an image to crate a posterized effect with the most control possible.
This not only allows for maximum flexibility, but also the power to customize where the levels break into different color, and also allows control over the colors.
Add a Black and White adjustment layer: Choose Layer > New Adjustment Layer > Black and White.
In the Black and White dialog box,click on the auto option, select one of the presets or use the sliders to convert the image to grayscale.
Please Note: The image appears to change to grayscale, the Black and White adjustment layer is only hiding the color in the underlying layers.
To Posterize the image: select Layer > New Adjustment Layer > Posterize.
In the New Layer dialog box, click OK and select the number of Levels (colors) you want to Posterize the file into.
If you posterize the image before changing it to grayscale you will get posterization per channel, this will give you three times the number of colors.
Refine the way the image is divided into the posterized shades of gray: On the Layers palette, double click on the thumbnail for the Black and White adjustment layer.
Adjust the sliders to change where the image breaks the levels of posterization.
To add color to the image: in Layers palette, target the top layer and select Layer > New Adjustment Layer > Gradient Map.
In the NewLayer dialog box, click OK. In the Gradient Map dialog box, click in the gradient area to access the Gradient Editor.
Create a custom gradient with the same numbers of color stops as posterization levels entered in the step above.
Each color will be mapped to the posterized image depending on its placement in the gradient.
You can assign a darker color to the gradient stops on the left, also lighter ones for the stops towards the right.
The advantage to using a gradient is that slight changes in the location of the color midpoint or color stops will change the colors that are added to the posterized image making different color combinations easy to try as they updated straight away in the image area.