Analyze the Mortar
Chisel out two or three small chunks of existing mortar for analysis. In a well-ventilated area -- preferably outside -- and wearing solvent gloves and safety glasses, pour a small amount of Muriatic acid into as many glass jars as you have mortar samples.
Note: Muriatic acid is a dangerous chemical. Please read all safety precautions before using. Be sure to avoid use around children, pets and plants, and consult local authorities for proper disposal method. If you feel uncomfortable, please send mortar to lab for analysis.
Gently, place one chunk of mortar in each jar of Muriatic acid. Do not splash. When the samples stop reacting to the acid, drain off the acid. The remnants will indicate the original sand color that was used.