I prefer using color liquid rather than color powder because it is less of a problem if it is a windy day. Some people say that you can get better color consistency by using a powder rather than a liquid because the liquid settles in the bottle, but first mixing with water solves that problem.
Using 1/4 of a bottle per 80lb bag of gray mortar makes a deep brown color.
Try adding Buff colorant to a bag of gray stucco.
One bottle will mix with two 80lb. or 60lb. bags of Concrete Mix. Available in Red, Brown, Buff, Charcoal and Terra Cotta.
Mix it with water to make a gallon of colored water. Measure out the required quantity of colored water and add that to the stucco mortar. Use half a gallon of colored water for each 80lb bag of mortar.