in my mothers place tiles over the bath has gone from white to black , and no they dont need to be cleaned, it an old extension single leaf cavity block wall they are on, but there is no dampness in the wall.

also tiles on splashback over cooker have discoloured fairly bad again no dampness,

does antone know what could be causing this and how could i prevent it happening again if i regrout them thanks

Sounds like mildew....any ventilation..?

Definitly ventilation is the major cause of discolouring,when out of bath/shower and dried always open window ! leave open for a while and room clears nicely and that keeps the mildew and all at bay.

How long have the tiles been on the walls?

tiles there 5- 8 years , ventilation definatly an issue , there is a window on that wall though , which id say is opened fairly often , guess i will put in a wall vent as well then

when you say "single leaf cavity wall", is it a "single leaf wall" or a "cavity wall", if it's a single leaf I'd expect a bit of damp to come through