If you are a die hard do it yourselfer, making dye for mortar is a rewarding and inexpensive way to color concrete or mortar. Concrete and flooring supply companies offer pre-made pigments that you mix with dry mortar before adding water. Make liquid stain and dye the mortar after it's dried to achieve this homemade method of dying concrete. The color won't be as dark as pigment mixed into the mortar, but you will still achieve a nice deep brown result.
Mix the muriatic acid and black iron oxide in a glass jar. The reaction happens quickly and the jar may warm up a bit to the touch. Refrain from placing the lid on until the reaction has stopped. You can store the stain for later with a lid if desired. You can find muriatic acid at hardware stores and iron oxide at pottery supply stores or online.
Add a teaspoon at a time to a jar filled with 2 cups of water to dilute the stain. The water should turn a bright yellow color but still be clear enough to see through. Too strong a stain will eat away the mortar. The light yellow color will produce a dark brown stain.
Apply small amounts of stain to the mortar, using a little stain but many layers with the paint brush. The purpose of using a little at a time is to prevent runners from forming and running down the concrete or brick. Let it dry.