How much pigment can I add to mortar before it affects the strength of the mortar?
Dosage limits for mortar pigments are given in the Brick Institute of America's Technical Note 8, "Portland Cement-Lime Mortars for Brick Masonry." Metallic oxide pigments should be limited to 10% to 15% of the cement content by weight. Carbon black should be limited to 2% to 3% of the cement content by weight. In most cases, color saturation is achieved before this point. Adding more pigment to the mortar is wasteful because it doesn't significantly change the mortar color. If the quantity of pigments used in the mortar is near or at the maximum limits, I recommend testing the physical properties of the mortar. The mortar should meet or exceed compressive strength, water retention, and air content requirements outlined in Table 2 of ASTM C 270, Standard Specification for Mortar for Unit Masonry.