I was inspired by the way you referenced installing pictures and did the same for myself.
Discoloured ceramic tiles.
After all the tile based discoloration has been successfully removed use a water soaked sponge or mop to get rid of any remaining traces of your solution.
The ctef certified tile installer cti program is the only third party assessment of installer skill and knowledge which is recognized by the tile industry.
Answer answer there are a number of variables involved in determining what is causing the discolored tiles and it will likely require some intrusive inspections to verify the cause and to determine how to remediate it.
Likely not so coincidentally there is a 4 strip of adhesive vapor barrier along the bottom row to the shower pan that correlate with the discolored tiles.
This is a job you can easily do yourself so give it a go first before you do anything else.
Cleaning your ceramic tiles.
The ceramic tile education foundation ctef provides education and installer certification for professionals working in the ceramic tile and stone industry.
While it may seem obvious regularly cleaning your bathroom floors is the most effective way to prevent tile discoloration.
Someone mentioned they are ceramic and absorb water.
Dirty stained grout can make an otherwise clean tiled area look filthy.
How to clean discolored grout.
You didn t indicate whether the tile is a glazed ceramic tile or an unglazed ceramic tile or whether either of those types of ceramic tile were porcelain body tiles.
To give your tiles a deep clean take a bucket of hot water and add a couple of drops of mild dish.