There is no hard and fast rule on HOW to digitally color. Digital coloring in infinite and there as many different methods to work as there are artists out there. I wish you the best of luck in finding the method that works for you.
Here are a few tutorials just to get you started. If you find some good tutorials, please share them in the class resources!
Coloring Line Art in Photoshop…
Simple animated painted style in Adobe PS:
Heres one for Procreate: