I want to give this app 5 stars. It's basic but does exactly what you'd want.
Step 1: load in an image.
Step 2: choose the resolution (slider with numbers)
Step 3: choose how many colors (it'd be nice to be able to swap colors)
Step 4: choose output resolution (clean upscaling)
Then, unfortunately there's step 5. Watch an ad... Ugh. Why? I've never before wished for a paid version of an app before now. If they add in a color picker (ideally), I'd gladly pay a few $ to unlock an add free version. Unfortunately, with the rather intrusive ads now, I can't give this app a rating higher than 3.