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Shower Tile Darker At The Bottom Few Rows Of The Wall

It could be water entering at the shower base and wicking up through the tile backer board.

First confirm the valve is not leaking water behind the wall. If not, then the water source is most likely from a faulty shower pan installation.

The shower pan is designed to allow for water wicking down through the grout joints. It is supposed to collect and direct this water to the drain. Sealing the grout and caulking any cracks are the first thing to try before redoing the pan. The easiest to apply sealer for glazed tile walls is "Same Day Sealer".

If caulking and sealing do not stop this, there is one more concept that has been suggested before tearing out the pan. This is to cut a 1/2" gap between the backer board and the floor, then retile. The gap will hopefully stop the water wicking effect. However, the water might still be entering and causing a different kind of problem elsewhere if the shower pan is not waterproof.