You can do it Poser 12. Usually what you want to change is Diffuse_Color. Click on the color block by the Diffuse_Color label and the color picker will pop up, just like on the Simple tab.

If it's a complicated shader, this might not work. Probably the easiest way to get rid of that shader "spaghetti" is to apply a very simple material. One you've made yourself. Or one copied from something simple, like one of the primitives that comes with Poser. You can save it to your library. Apply it, then adjust the diffuse color.