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.
Discoloured ceramic tiles.
Someone mentioned they are ceramic and absorb water.
The ceramic tile education foundation ctef provides education and installer certification for professionals working in the ceramic tile and stone industry.
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.
I was inspired by the way you referenced installing pictures and did the same for myself.
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.
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.
While it may seem obvious regularly cleaning your bathroom floors is the most effective way to prevent tile discoloration.
Cleaning your ceramic tiles.
To give your tiles a deep clean take a bucket of hot water and add a couple of drops of mild dish.
Step 5 prevent discoloration from reoccuring.
Removing the stains without replacing the grout requires some effort but is less intensive.
How to clean discolored grout.