I would guess that Mark is right, you are dealing with a pigmented mortar.
I would suggest getting some colors from a masonry supply, and doing some
tests on a mortar board accurately measuring volumes of each pigment in your
mortar mix.  Factors in the make up of the mortar, such as the sand, lime,
sometimes portland(grey or white) and any other admixtures, will come in to
play as well.  The level of accuracy desired should determine the effort and
expense involved.  Sometimes just mixting some stuff together is close
enough, and others require in depth research and analysis.

Rob Cagnetta