They do a polish remover but not sure on how it works as i do not use that particular brand..you can then use the grout colourant and then re-seal with the same sealer as used before..making sure you follow all the directions very carefully...if you are unsure then please DO get this done by a pro..
I heard that tile giant stopped using granfix as there where too many come backs with there products. Doubt they will have this issue using a brand like Mapie though