My tiles are becoming darker.

Quite simply... No, they're not. Tiles cannot change colour. Tiles are an inert product like stone. They do not fade in sunlight and neither can they tan, or turn a darker shade.

If a tile is contact with water however, it will absorb water. When the body or biscuit of the tile absorbs water it will appear darker. All tiles absorb moisture to some degree, depending on the type of clay biscuit. And then, just like wet chalk, or a sponge, the biscuit will appear to be a darker colour or shade. Even porcelain will absorb water, by definition less than 0,5% of water by volume. If the glaze on a wet tile is white or a very light colour it tends to be translucent or see-through. The darker background biscuit will then make the tile look darker than other dry tiles.

Water damage that originally came from above but then seeped below the tiles and caused discolouration and in this case also caused the tiles to lift.

In extreme cases, even when the tile has completely dried out, the biscuit can be stained by impurities that have entered with the water and the colour of the tile will appear to have changed.

Water absorption is really the only way that a tile can appear to have changed colour. If water has penetrated behind the tile, or is caught behind the tile it will soak into the biscuit of the tile. Water can penetrate behind the tile through holes in the grout, missing grout, or poorly applied silicone joints. Especially, if there is a partly blocked drain and slow draining waste and any silicone joints in the internal corners are not intact. This is particularly common in wet areas like showers.

It is not a problem with the tile, but the installation, or perhaps a serious water leak.

If you notice that the tiles in your wet area, especially in the shower recess have changed to a darker colour, the situation needs attention as soon as possible. It is an indication of a problem that can cause long term structural damage if left unattended. Call a plumber to test for a leak in the water supply system.

Check for any obvious visual faults or weak points in the grouting and silicone joints. If it appears suspect, remove the grout in the affected areas. Dry the tiles out slowly, using a fan heater on low. This should bring them back to normal appearance. Regrout the tiles using grout mixed with a 'booster.' Apply silicone to all corners in the shower.

It can also occur in apparently dry areas. If the tiles were laid before the concrete underfloor was dry and properly cured moisture will percolate upward into the tile body. This will cause the tiles to look darker. In fact they are simply waterlogged. Even a small amount of water can have this effect, especially with very pale tiles.