When: only once you can construct and pose a character from imagination on paper — roughly the end of Phase 4, into Phase 5. Digital drawing is a separate motor and software skill; layering it on too early spends hours you don’t have to spare.
Once your layer workflow is comfortable, learn flatting — filling each area of your linework with a flat, solid color on its own layer before any shading. It’s the standard first step in digital comic coloring, and the difference between a coloring session that takes 20 minutes and one that takes 3: this Procreate walkthrough shows the selection-and-fill method.
Flatting is mechanical; picking colors that actually read is a separate skill most free “how to color” tutorials skip. Working comic colorist K. Michael Russell teaches color theory alongside flatting for free on his own site and YouTube — the same lessons he sells in his paid courses, offered as free trials. The Sequential Artists Workshop’s free “How To Color Your Comics” is a shorter, practical companion: get your values right before worrying about which exact hue you picked — readable line art beats a pretty palette every time.