My brother taught me a trick from his body shop days to help soften the edge on a color line.

When you paint, spray a test piece too. When you can touch the test piece and you leave a finger print but the paint has skinned enough that it doesn't stick to your finger, that's the time to remove your masking tape. The paint will self level and instead of having a sharp edge where the masking tape was, you'll now have a much smoother rounded edge. Not absolutely ridgeless but much smoother than if you allow the paint to really set up before removing the masking. This works with any paint but much better with automotive paints.

When I asked him about the absolutely smooth color seams like on a custom motorcycle gas tank he said each color line is masked twice so the colors don't lay on top of each other. Then it's wet sanded to make everything absolutely smooth. Then it's shot with clear coat.

Dave