It's nearly impossible to match concrete, but following these steps will get you the closest color match that you can get without having to redo your entire cement project.
If you're trying to match concrete, the first step you need to take is to search for the different color ranges of concrete in your area. It is nearly impossible to perfectly match concrete, but you can find the best color to match the existing concrete using this method.
After the new cement sets, try rubbing dirt into the area and then rinsing it off to see if you can get the colors to match a little better. (It is possible that over time the new concrete will be a closer match to the old concrete, but it's definitely not a guarantee.)
If the colors of cement are still very different from each other you can try an epoxy overlay. With an epoxy overlay you can cover both the old cement and the new cement for a better color match. There are many different types of epoxy overlays available.