While matching mortar color may seem like a simple proposition on first glance, there is a lot more to this process than meets the eye.
The base formulation of the mortar affects the color of the mortar.
Failure to match the development of the original mortar can make it nearly impossible to get an accurate color match for the repair mortar, and will likely crumble or cause deflection of the brick faces.
The recommended protocol for accurate mortar matching involves the use of mortar analysis. By testing samples of old mortar, the formulation of the old mortars and how they perform can be determined. A new mortar can then be prepared that matches the formulation of the existing mortar, and be adjusted to the color tones to create a perfect mortar color match.
During testing, the amounts of Portland cement, lime, and sand used in the existing mortar can be determined, as well as the color and size of graded sand particles. Sealers, pigments, dirt and contaminates, or fine particulates in the mortar can also affect color or performance. All of these details will affect the color, texture, performance, and appearance of the repair mortar, and will also determine the durability of the repairs.
The mortar formulation analysis can also determine the water absorption/deflection, compressive strength, and flexural strength of the mortar, and in turn, can determine if the repair mortar will bond to the existing mortar or carry the load bearing on the mortar.
Importantly, when mortar is not properly matched to the formulation of the existing mortar, it can cause more damage to the masonry than was present before the repairs.