Has anyone any experience of grout statining dyes,

I laid a kitchen floor last year, despite my recommendations the customer insisted on white grout, To protect it I sealed it with HG SUPERPROTECTOR which has done the job and it is stil pristine white, however, The lady of the house has decided that she would like it changing to gray to match the worktops, I dont want to rake it all out and redo it and have told them I wont even qoute for it.

Will a grout staining product work, the tiles are ceramic, Im guessing that the sealer will stop it from penetrating the grout, is there a remover product?

Any ideas welcome.