HG grout cleaner concentrate removes persistent dirt from grout. Any yellow or brown dirt that remains after cleaning is limescale, which can be removed with HG professional limescale remover (hagesan blue).
For black deposits on grout, please use HG mould spray. Dilute (1:4) HG grout cleaner concentrate with lukewarm water.
Then apply with a hard broom or a scrubbing brush. Leave it to work, scrub well and rinse with water.