Separating Two Wall Colors

Make a light mark on the wall where you want the two colors to meet.

Paint the wall normally, with the roller in flat areas and the brush in joints, with the first color, in the area where you want it. Don't worry about going over the line a little bit. The second color will cover it. Allow paint to dry.

Use the level and pencil to make a precise line where you want the edge. Draw the line onto the first color of paint you just put down. The level can be used vertically or horizontally.

Apply the tape, one side aligned with the drawn line and the tape covering the area you just painted, leaving the area for the second color uncovered. Rub the softly to make it adhere better.

Apply acrylic matte medium, using the brush, in a light coat, directly onto the tape on the edge that faces where the second color will go. Overlap the medium onto the wall a little. Do not paint onto the first color. The tape is sufficiently wide that this won't be a problem. Allow the medium to dry, usually 30 minutes.

Paint your second color in the same manner as the first. Paint over the tape at the edge with the brush, but don't get any onto the first coat.

Pull up the tape to reveal the clean edge separating the two colors.