Hygrophanousmushroom tissue (usually the cap) changes its degree of transparency depending on how much water it has absorbed. Since it both absorbs water and dries out unevenly, this means that the cap will appear to be different shades of the same color in different places, depending on the tissue's degree of saturation.
Another effect of this is that where the cap is more transparent, the color of the gills shows through more. In this Coprinus, the gills are black from spores, and this shows through more near the edge of the cap, where it is more transparent.
