I needed to repair a crack in a chimney and wanted the color of the new mortar to match that of the existing mortar. I had heard about using acid concrete stain for this purpose, so I called the local paint store. They could mix only gallons. I started researching the idea of making my own concrete stain and found this web page on the topic: finishing.com/242/29.shtml.
From some of the conversations on the web page, I gathered that I could put two parts muriatic acid (a common cleaner for concrete and mortar) into eight parts water to make a solution for concrete-coloring pigments. Wearing rubber gloves and eye goggles, I used a glue syringe to inject 4 cc of acid into 16 cc of water. (Always add acid to water to avoid splashing acid.) After that, I just needed some pigment.
Years ago, I read a tip in Fine Homebuilding about Mixol universal tints. I bought the basic kit, which comes with a color chart showing the different hues that can be produced with the 10 concentrated pigments. The lightest shade of #23 looked exactly like what I wanted, so I put about four drops of color #23 into my 20 cc of acid/water mix and tried it on concrete in my shop. It was a little too light, but about six drops more gave me the color I needed.
If you try this, follow all the instructions on the muriatic-acid container, and be sure to do a test in an out-of-the-way place.
Don Mathis, Macomb, IL