Blender color reduction I'm trying to post-process my render to achieve a low color depth look. For some reason the image becomes darker and the render do not really matches the look I want. 

Exporting the not post-processed image and using the same algorithm on another program results in the look I want but it does not in Blender.

I would like to achieve the right image look using Blender. That will improve my working speed for my project because I won't need to use two programs.

How can I make Blender's compositing result match the right image?

Thanks for your help.
Create stairs in an "RGB Curves" node. To make the line straight select "Vector Handle" on each dot. Hold CTRL and click on the line to create new dots.