hi, i had exactly same problem after tiling my bathroom, light plain porcelin wall tiles from b&q (i know i know) were covered in a ingrained stain from the dark charcoal grout. tried the nail varnish remover to no avail.
after searching on here i evetually bought something from Topps by fila called-grout residue remover and it did exactly what it said on the tin!
takes a while to work mind, dont know if its a bleaching process or not but after initial application and scrubbing nothing happened. much disgruntled i left it for while went to work and came back to find it had completely removed a said staining.
it does as some have mentioned tend to bleach the grouting and its pretty impossible to apply without touching the grout but small price to pay and that can be relatively easily resolved.