Layers are a feature of many digital painting programs. They may seem a bit of an odd concept if you are not used to them, however they are invaluable once you really get into coloring.
The best way to think about layers is to think of lots of transparent sheets layered over the top of an original image. You can add all sorts of decoration, detail and textures to these layers, but underneath the original image stays the same.
And this is how layers work on digital painting apps. You can add layer after layer, changing and refining the coloring of your grayscale page. If you don’t like a layer, just delete it and start again. One of the joys of layers is that if you mess up one part, you haven’t ruined the whole artwork.
Another exciting feature of layers is that you can blend the layers to create different effects. If you were to color, using a solid color which is 100% opaque, it would hide completely your base image. Therefore you’d either change it to a transparent color, so your base image shows through – or you can use one of the blending options. The best way to demonstrate this is by video. I am using MediBang Paint Pro in this short video:
You can see that each blending option has a different effect. Try different ones and see which effect works best for you.