Whilst the glazed finish on glazed ceramic tiles is impervious to water, if there is a leak or excess moisture, the tile may become wet from behind or underneath. This is often more evident in a harsh, wet environment like a shower recess where wall tiles are often made with a softer biscuit (base).
Because the glazed surface is not unlike a glass finish, if the tile biscuit is wet, the tile will appear darker on the face of the tile.
Simply, water is managing to get behind the wall or under the floor and soak into the tile.
This can be due to a number of reasons including:
See our list of Find a tilers here
Regardless of your final solution, install fluid aprons around your taps. These nifty little gadgets are available from your local Beaumont Tiles store.
Once the cause is fixed and the tiles are given the opportunity to dry naturally, the dark shading should disappear.