Clean your doormat of debris, or use a new one.
Place long strips of tape at parallels (about 5 in each direction) to create diamonds on your mat.
Space them irregularly to create different sizes. Choose a color scheme. We went with teal, blue, and grey. Dab the paint into each shape. Try not to let the same colors touch.
Allow the paint to dry for at least an hour before pealing back the tape. Acrylic paint will harden into a plastic when dry, and holds up well on a doormat.
Enjoy your "new" mat!