Cleaning the dirty grout on kitchen tiles.

Postik

Apparently bleach weakens grout and can also make it more yellow.  However, at times I've had to resort to it as no other method has worked.

I've found vinegar doesn't seem to touch grout when it's really dirty.

You could try a steam cleaner which gets very hot and might help to melt some of the ingrained grease.

Most likely though, the grout pen you used is probably the only option that will really work, aside from re-grouting.

jenni_fer

After years of repeated cleaning using all the tips above, I knew mine was clean but it was most definitely yellow.
I bought Forever White Grout Reviver on a whim and it was amazing, takes a bit more effort than a pen but looked like we'd regrouted! I ended up doing the whole house!

twopenny

I used to use shoe whitener. 
Was used for tennis shoes and trainers before they became washable.
No idea if you can still get it. I was about to try recently before life went pearshaped.
I used it in a 1920 fire place that the boiler burned coal and stayrd white. Ditto lasts ages in a shower or kitchen.

Chloe_G

twopenny said:

I used to use shoe whitener. 
Was used for tennis shoes and trainers before they became washable.
No idea if you can still get it. I was about to try recently before life went pearshaped.
I used it in a 1920 fire place that the boiler burned coal and stayrd white. Ditto lasts ages in a shower or kitchen.

Took me back!  I used to love whitening my plimsolls!