I combined designer Miguelito
's PixelBoy pixelation image effect with designer Rogue Noodle
's palette-ramp GBCamera effect to create a single effect that can pixelate and set a new palette to your game easily. I also improved upon Mig's original code by ensuring the pixels are as close to square as possible no matter what resolution and also added the ability to base pixelation on a division of the user's resolution so that it is consistent and also perfectly square if using particular even divisors. I also combined it into one pass so that it should be more performant than using both individually.
You can set ramps via dragging your desired color ramp and checking the 'Use Ramp' button. These ramps are basically your palette and you can design them to be however many colors you want. It can be really useful even without the pixelation.
Just throw the script on your camera, probably preferably earlier in the order of image effects, but that's not necessarily required. The effect may interfere with other image effects, so let me know if you have a problem, and I can try to sort it out.
Thank you to Mig and Rogue for allowing me to work with their effects!
Made in Unity 5.3.1.