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.
Discoloured ceramic tiles.
The ceramic tile education foundation ctef provides education and installer certification for professionals working in the ceramic tile and stone industry.
How to clean discolored grout.
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.
Dirty stained grout can make an otherwise clean tiled area look filthy.
Removing the stains without replacing the grout requires some effort but is less intensive.
Cleaning your ceramic tiles.
Someone mentioned they are ceramic and absorb water.
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.
While it may seem obvious regularly cleaning your bathroom floors is the most effective way to prevent tile discoloration.