Sgraffito

Sgraffito means "to scratch." It's drawing on your pot.

Step 1 — Brush two or three coats of colored slip (liquid clay) onto a pot that's just firmer than soft leather-hard.
Step 2 — Let it dry until it's no longer shiny.
Step 3 — Draw your design with a needle tool, scratching through the slip to the clay body underneath.
Step 4 — Brush away the crumbs with a soft dry brush. Don't blow on it.

The contrast between the two colors is where the magic lives.