In his "Church of Light" Tadao Ando expertly uses contrast to depict a glowing cross on the wall of this church. The concrete is contrasted with the non-concrete (negative space) for a very dramatic effect.
This my example of a bad use of contrast. I went on a hike in the Muir Woods with this map that is provided to you for a donation of one dollar. I found this map very hard to use, due to lack of contrast between the different lines. (trails, roads, contour lines, etc.) In addition to this, there was a "you are here" arrow pointing to the location where the map was received. That is really stupid because once you leave, you are not longer "there". However, the later problem was not due to lack of contrast.
