If the tiles have been around for a while your tile restoration may just include a good cleaning to restore their shine.
Discoloured ceramic tiles.
I was inspired by the way you referenced installing pictures and did the same for myself.
To give your tiles a deep clean take a bucket of hot water and add a couple of drops of mild dish.
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.
How to clean discolored grout.
Removing the stains without replacing the grout requires some effort but is less intensive.
This is a job you can easily do yourself so give it a go first before you do anything else.
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.
The ceramic tile education foundation ctef provides education and installer certification for professionals working in the ceramic tile and stone industry.
Dirty stained grout can make an otherwise clean tiled area look filthy.
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.
Cleaning your ceramic tiles.
Someone mentioned they are ceramic and absorb water.