https://www.cutoutandkeep.net/projects/how-to-get-perfect-paint-lines • Posted by Corey
Make a template to mark the width of your stripes. Tape them off with blue painters tape using a level as you go to check your work.
Once you are done taping, press the tape to the wall to get rid of as many little bubbles as possible. NOW FOR THE TRICK... Paint along the lines where your new color will be with the same color that is already on the wall and let it dry. This will seal the tape and if there are any little spaces for paint to seep through it will be the right color.
Once that has dried, paint your new color and peel the tape while it is still wet.