How to make pixel art from a picture (only with numbers for coloring)?

Question

Graphic arts

Good afternoon. Interested in the question of how to convert a picture into a white canvas with a grid on which numbers will be applied, which will correspond to a certain color?
And, perhaps, another online service where you can upload such pictures, set colors and decorate.

Example, as in Android games:

screenshots

Only there the prepared image is immediately given, and I first need to convert it into such an option (like on the second screen), which then could be decorated