"is there a way to quickly change the diffuse color, and also use a preset pallet?"

SnarlyGribbly’s free EZmat for Poser (linked above) has a "Randomizer" script in it. This randomises the diffuse colors, to give you a Clown Pass. Not quite what you want, but near.

It sounds like you want what Hollywood calls a "MATcap" - to have Poser automatically capture a material's average color, and then replace with the material with a simple diffuse using that averaged color. Such a thing would be useful for comics makers, and we can only hope to see a MATcap "insta-comic" button added to Poser 12's comics features. It would be even more useful if we could then apply a palette-shift to the results, so as to reduce the applied colours to stylized shades of gray (for instance).

Stripping materials, and then adjusting diffuse colors down to a limited pallete - that sounds like you are making comics with Poser.

If you are making comics, you should be Preview-rendering to separate inks and color layers. Then combining these renders as layers in Photoshop, first knocking the white out of the inks render and then dropping it on top of the color render.

Doing a comic this way lets you filter the colour render, without ruining your inks. You can do things like using a "Replace Color" Photoshop Action on the color render, or third-party simplification filters such as Topaz Clean. Again, this may approximate what you want to do.

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Learn the Secrets of Poser 11 and Line-art Filters.