Cement mixes are formulated and packaged at plants across the United States. However, not every company uses the same formula, or even the same ingredients. Even contractors in the same area may use different brands. Because of this, new or replacement cement will almost certainly not initially match the tone of the older cement. There are some ways to artificially age and darken new cement, but matching 100 percent is difficult.
Purchase several test bags of cement and mix small batches in small buckets. Label the individual cement samples and allow them to dry completely. Place the samples on the existing cement and pick one that looks the most similar. Pour a coating of the new cement over the top lighter surface, trowel it smooth, and let it harden.
Place a layer of fresh topsoil over the new cement and wet it with a garden hose. Let the soil stay there for a few days, and wet it repeatedly as it starts to dry out. Remove most of the soil with a shovel, leaving only a small amount. Walk on it repeatedly to darken the cement. Though it may not match perfectly, it should darken somewhat.
Pour a small amount of concrete stain in a small bucket and add water to thin it. Test the stain and let it dry on sample concrete to see if it matches the color of the existing cement when dry. Add more water or stain as necessary to get it close to the same and then brush the stain on the new cement.