DIY painted rock fireplace. I updated our rock fireplace last weekend. It was 70s brown stone and I wanted to lighten it up to better match our walls, which we were painting grey. It only took about 2 hours and less than $15 to do. I made a simple mix of grey acrylic paint (found at Michaels for .75 ea. (took 3 bottles total) and a little water. Sponged it over the brick (left mortar dark grey) and sealed it with a $10 gallon of masonry seal (found at home depot).