Depending on how porous the brick is, and color matching ability, you have a few choices: a ready made masonry sealer and stain the easiest way, a clear masonry sealer pigmented with universal paint tints, the old fashioned stone masons way dry earth colors, boiled linseed oil, plus muractic acid or combining new with old masonry sealer with dry earth colors.Your friends suggestion of using a roller to keep the stain off the mortar will not work. If a masonry sealer is used it should be applied by daubing with a well rung out rag. If a paste with dry earth colors plus masonry sealer is made, a small stiff brush like a flux soldering brush may be used.Again the easiest way is using a masonry sealer and stain. A possible problem is that some brick is fired longer and it becomes more impervious. Flick a few drops of water on to the dry brick. If it disappearers readily use masonry sealer and stain. If it does not disappear readily, try a masonry sealer and stain, but be ready to move on. If more color or shading is required: from a paint store purchase universal tinting colors and add to the masonry sealer.If the masonry sealer and stain does not “take” readily: from a masonry supply purchase a one pound box of “limeproof red” red dry earth cement color. Make a paste of masonry sealer and the powder and apply with a small stiff brush or a rag. When making the paste strike a balance between the stiffness needed for a vertical surface and fluidity for penetrating ability. When the paste hardens somewhat rub all of the excess off with a stiff scrub brush, followed by a rag.The last resort is the old fashioned way: make a stiff paste of limeproof red plus a small quantity of boiled linseed oil. To this add straight muractic acid, until it becomes a semi-fluid paste. Do not touch the paste with your bare hands. Apply with a small stiff brush, wait, rub off the excess with a stiff brush, followed by a rag. By varying amounts of colors, linseed oil, acid, and surface contact time a great deal of staining control can be achieved.Remember: wear protective gear, including rubber gloves, safety goggles, and a dust mask when handling dry earth colors. And read all labeling before purchasing products.