Its not that >>459331 is "wrong" or that his techniques aren't valid or useful but the result is clearly not the same, mostly because of using a "stencil" approach...the Binder posters definitely used masks to create certain effects but there's little if any stencil- like keyline like in that anon's example.

Binder's style is a prime example of *posterization*, which is more about reducing the number of colors used within an image...it can be done manually or photographically and is in many cases an inherent effect of the printing process used and how the colors are separated.

To do it digitally, tweaking brightness and contrast and exposure can help to eliminate more subtle gradations and give the feel of reduced color depth, but it's not the same as what you get with a "posterize" filter or adjusting color depth downwards. When done digitally you can get weird banding artifacts so blurs can help reverse that and still keep those colors reduced but not to the point of looking like stencils.

Most gouache effect filters are essentially doing posterization with some saturation change to mimic it's more opaque nature.

Picrel is not identical to the look of Binders posters but as you can see limits the color pallette and simplifies detail the same way without having any stencil-y keyline