Colouring pages with Procreate

How can I use Procreate to separate a line drawing from its white background?

1. Select the layer you want to prepare.
2. Copy the layer.
3. Create a new layer on top of it.
4. Fill the new layer in with solid black.
5. Create a mask layer for the black layer.
6. Paste your image to the mask layer.
7. Invert the mask layer.
8. Merge mask layer and black layer together.

You should now have a new layer on top of the original image that consists of the line drawing on a transparent background.

Keep your new layer as the top layer and add as many colour layers you want underneath it. 

Now you can colour in the image any way you want and the line drawing will always stay on top and unaffected by your colouring.