Credits to a graphics tutorial book because I learnt the skill from there.

App used:
Picsart

1) open Picsart, go to the draw function and choose your picture

2) tap on the icon beside the trash can and add 2 layers, one above the picture and one below it

3) draw an outline of your picture (I recommend using the thinnest brush aka thinness 1) & erase any mistakes
This is so you get a png/ render of your picture

(My drawing sucks but it should be something like that?)

4) colour your picture (colour it on the top layer) and make the top layer darken

TIP: use a reference for the colour you're using by using the dropper icon after placing it on the picture and deleting it

IF YOU DON'T WANT YOUR END PRODUCT TO BE A RENDER/PNG:
Skip step 3 & only add 1 layer above the picture

End product

-Sherri