Hi Mate
I use detox Mould & Mildew remover over all the grout and sealant and leave it too whiten out, works 8 out 10 times.
Usually in our situation it's dirt in the grout or rotting bodyfats (that lovely orange colour)
Sometimes it's so bad that only a replace will do. I do about 10 of these a month, so if you need it give me a shout.
Regards
Martin