We often place thin concrete overlays to repair surface defects such as small spalls and scaling. Because it hasn't weathered, the new concrete surface is often a different color than old adjacent concrete. The new surface is usually lighter. How can we reduce the color difference?
Some contractors use one or more muriatic acid washes to artificially weather concrete. Others suggest first etching the surface, then washing it with a mixture of carbon black and water. A third alternate is using a diluted black or brown concrete stain to darken the color. Make some test blocks with concrete from your normal supplier and experiment with several methods. Use the blocks as a color key for matching the older in-place concrete.