Mortar is the glue that binds bricks together, making them one of the sturdiest and most water-resistant construction materials used today. Mortar comes in a wide variety of types depending on where it will be used and is available in many colors to suit your design needs. If you have a fireplace, stone wall or other bricked area with mortar that is not to your liking, there are two basic ways you can change the color of the mortar: applying a muriatic acid to the surface of the brick or re-applying new mortar.
Apply a 10 percent solution of muriatic acid to the mortar with a small paintbrush.
Allow the acid to remain on the mortar for about five minutes or until the acid has stopped fizzing.
Rinse the area and allow it to dry to see the color change. Muriatic acid will lighten up the mortar slightly. If the area is not lightened up enough, wait a few days and apply a second coat of acid.
Apply New Mortar
Use a chisel and hammer to remove ¼-inch of the existing mortar from between the bricks.
Mix up some mortar in a large bucket by following the directions on the package. Once the mortar has been mixed, add a powdered pigment into the bucket to tint the mortar the desired color. Test a few samples of the mixture on a mortar board to determine if the mortar is the desired color before you apply it to your brick.
Apply the mortar to the gaps between the bricks with a trowel. Scrape off any excess mortar with the trowel and allow the surface to dry thoroughly. Your mortar should now be a different color to match your design needs.