We had portions of the "city owned" concrete sidewalk replaced in front of our home and wanted to have the new slabs match the older slabs in color. A city worker noted the older slabs had changed color mostly due to discoloration and staining from the falling and decaying tree leaves, and suggested covering the new sidewalk portions with the season's leaf fall. We have called this our "accelerated color" project. It appears that most of the coloration is a chemical process that occurs as the leaves lose their green or yellow color and turn a dark brown. We have also soaked the layers of leaves and worked to keep them wet during days without rain (on the theory what this promoted decay and a transfer of chemicals from the leaves to the concrete). Once the leaves are a dark brown, it does not seem any staining occurs. We have gone through three "coverings" and while there is still an unevenness of the staining, we are totally pleased with the progress of over the month. The slate gray of new concrete is gone and we are well evolved toward the brown tinge of the older concrete. It would appear that grinding up the leaves in a shredder (while still green or yellow) and evenly spreading the mix on the new concrete might yield a more even and faster staining. Perhaps others will share their "natural staining experiences" and try the improvements I've suggested.