Using the same color, paint along the tape edge. This seems strange but, there will always be some bleeding under the tape. By deliberately painting against the tape, you seal the edge with the first color, allowing it to bleed under the edge, so the second color can't do it. The edge of the tape becomes the edge of your line.
Make sure the lower edge of the paint feathers softly away so you won't see a thick edge of paint later on.