I really like the effort here, the cartoon style is cute and endearing and I can't wait to see where you go with it! Shading with MS Paint can be tough, and using Photoshop is not always easier. It's definitely more accessible, but then you are trying to get into more stuff than just shading and coloring rules.
I am not an expert when it comes to coloring digitally but I do have a couple recommendations:
1. Learn your program better, the more you know it the better you can get it to work for you! Look at GROUP and LAYER functions specifically, and find what coloring method works best for you. Some people (coughmecough), like to have an original sketch scanned in and then put layers on top on multiply layer settings. Others, have a transparent line layer and put color underneath it, etc. Find what works best for you!
2. As for the coloring, try staying away from just solid colors. Use multiple shades of the same color for shading, so that way it looks more natural, if you look at different pixel drawings for example, it really shows how there are different layers as it gets towards the darkest part of the shade. Blending is also cool too to keep the shades natural, so lightly doing that in between the different shades might make the transition less blocked.
It looks like you are trying to do cell shading though, so for that I would just make the shading less "rounded" and more sharp, like the shading on Kairi's hair that's extending into her cheek. Instead of making it round, do the shape of the hair instead (which comes down to a triangular point).
That's all I got for now, keep up the good work!