Import and protect the outline
Bring in a high-resolution PNG or PDF, then keep the original line art untouched on its own layer. Set the line layer to Multiply when necessary so color beneath it remains visible, and duplicate it before making structural edits.
Color on organized layers
Create a layer below the outlines for flat color and additional layers for shadows or highlights. Use ColorDrop or selections only after checking for gaps in the line work; a small opening can send a fill across the page. Name layers by subject so later edits stay quick.
Export a finished copy
Keep the editable Procreate file, then export a flattened PNG or PDF for sharing. Check the image at its actual display or print size, since tiny edges that look perfect while zoomed in may disappear in the final file.