EXT wrote:

The idea behind acid is that it won't harm the glass glazed surface of tiles
but will eat away at the mortar or grout to remove it from the surface. It
will eat some of the grout between the tiles, but you shouldn't use much
acid and it must be diluted with water to ensure that it is not too strong
and does damage to other things and surfaces.

ALWAYS add acid to water NEVER the other way round. The way it was explained
to me is: when adding acid to water, the first drop of acid to hit the water
will be immediately diluted along with the rest of the pour; if you do it
the other way round, the first drop of water that hits the concentrated acid
will immediately explode in steam, along with some of the rest of the pour
and this can hit your eyes, face, skin and other items nearby doing some
serious damage.

Colored glaze, which might be unusual for tile but common on other
ceramics, is colored with metals. Don't know how muriatic would handle
that, but my only experience with muriatic and tile was hearing somebody
else's sad story. Perhaps unglazed tile. Our tile and grout was
installed by contractor, cleaned well. He advised, and we did, washing
after few days with vinegar/water. After grout cured a while. Even if
muriatic doesn't harm the tile, using it indoors can harm a lot of other
stuff.