Coat the risers and woodwork with white paint. Tear off a fist-size piece of natural sea sponge. Wet the sponge with warm water, then wring it dry. Lightly dip the sponge into the blue paint, then tap it onto the paper towel or paper plate until most of the paint is removed.
Gently dab the sponge on to the stair risers, repeating until the color becomes faint. Load the sponge with more paint, and repeat the process.