Re: advanced posterization

That's right, you can select a range of colors and Photoshop will ask for the first and last ones. Then, it will create a gradiente, as you said. You can also select a single color, for example, if you want the first and last ones to be black and white, you can create your gradient and then change the first and last colors. Sorry my post was not 100% clear from the begining, english is not my native language.

This process is, as many others in photoshop, a try and error one, but you can get some funky looks with it until photoshop integrate a similar filter to the Colorama in After Effects.