Making clean paint lines between two colors doesn't have to require a steady hand or special equipment. This technique is very simple and requires only paint, brushes and masking tape. This time, however, you will be controlling the bleeding paint and using it to create crisp lines that precisely follow the edge of the tape.
Step 1: First Color
Step 2: Taping
Step 3: Bleed Line
Make sure the lower edge of the paint feathers softly away so you won't see a thick edge of paint later on.
Step 4: Second Color
Step 5: The Reveal
Tah-dah! Crisp, clean paint lines!
(I hate adding a caveat but it seems warranted here: I haven't had any problems with the line when removing the tape after the second color has dried BUT other people I know have. It has to do with paint setting up and binding to itself. So, if you cannot pull the tape while it is still wet or at least soon after it dries, you might consider using a craft knife and a straight edge to score the line before pulling the tape.)