I had some bricks replaced. Unfortunately, the mortar is really light. How do I stain it a darker gray, so that it matches the original mortar? Is there a product that I can buy at a big box store?
Universal dyes might do what you want. The trick is having the patience to test them to final color at dryness Here's one color brand (no association, haven't used them)
The source here (National Park Service) recommended against them (dyes), and they might diverge in color over time. They suggested cleaning the surrounding area.
Diluted muratic acid brushing/power wash at a months's time would blend the old and new effectively (use protective gear/old cloths/gloves/goggles) hose down landscape plants beforehand)
Here's a tutorial for cleaning mortar off brick from Fine Homebuilding
We had the same problem. Our chimney had some repair work done to it and the new mortar did not match the old mortar color. We fixed it ourselves by using a product we found online called Instant Mortar Match. The website is InstantMortarMatch.com. They sent us a color card, so we could pick out the best color to use. Then we just brushed it on the joints. It was simple to use and it turned out great. When we called the company we asked about muriatic acid, they recommended against using muriatic acid because it can eat away at the lime in the mortar.