In this picture of the front yard we can see the contrasts between the concrete curbs around the flower beds and the darker bark mulch now that it is installed. The curbs are stained a light sandstone for a couple of reasons. Besides the contrast, they will withstand showing the hard water stains that will come from the sprinkler system hitting the edges of the flower bed.
While the curbs are good the way they are, they would have also looked beautiful in a darker complimentary tone. However, as I mentioned, eventually the darker color would have shown the water stains more clearly.