Hey, I really love this shader (and it still works with Unity 6!) ...but I never figured out how to create custom lut images to change the color style. Maybe someone can give me a tip :-) Thanks!
Sorry for late reply, shader contains pdf with instructions inside of it but basically - grab lut_stock.png palette. In photoshop - apply indexed color with palette of your choosing to it (no dithering). You're done. You can extract these from any paletted image in photoshop as well as well as from old games which provided their own palettes or even from people who make them like arne.
There is also a possibility to do more colors than 255 to match 16 bit platform specific stages but it requires some coding, me personally - i did such a tool through processing software way back in the day, but i can't find it.