I could use dye but would prefer an original carpet in good nick, side kick plates as well.
Get the carpet professionallly cleaned, it will come up a treat. There would be no point trying to dye a carpet that is dirty. Professional carpet cleaners use products that can remove stubbon stains and grime.You mentioned dye.... Do you know of a dye that is suitable? I've found it difficult to select a dye as I don't know if the carpet is nylon or a mix. Pure nylon would make if virtually impossible to dye unless the carpet was placed in boiling water.
In my head, spraying on something waterbased with a spray bottle from a garden centre would be ideal.
Has anyone done it? Its not a colour change, just looking to freshen up the grey carpets.