1.Draw, and create original work using am HB pencil or a dark marker pen.
2.Take a photo of your original work and save the image you created to your computer or email into yourself. You may also scan the original work and save it to your computer.
4. Once you have the image on your computer, you can open the file with your favorite image editor * that supports layers. Krita is a great free software that will have everything you need: https://krita.org/en/
or if you need a browser based solution you can use https://www.photopea.com/
which is again totally free, very good and an alternative to Adobe Photoshop... also, as far as we are aware Microsoft paint doesn't support layers hence my suggestions.
5. with your line work selected in the layer panel, lock that layer and add a new layer underneath your line work.
6. go directly to the paint brush tool, choose your initial colors and paint on the new layer you created, (the one directly under your line art) this will add new dimensions to your work! You can add as many new layers as you want to, coloring each.