Bring in a clean file
Use a PNG for the simplest import or rasterize a PDF at the canvas size you need. Avoid stretching a small image, since enlarged line art becomes soft and fills become harder to control. Duplicate the original layer before you begin.
Place color beneath the lines
Keep the outline layer at the top, set it to Multiply when appropriate, and add a flat color layer underneath. Use ColorDrop or selections for enclosed regions, then add shadows and highlights on separate clipped layers. Zoom out often to check the overall palette rather than judging every fill in isolation.
Keep the editable version
Save the layered Procreate document, then export a flattened PNG or PDF for sharing. This keeps later palette changes possible without asking the line art to do two jobs. For pages made from scratch, see the guide to making a page in Procreate.