Long story short, cream carpet already fitted. Dropped cup of coffee on it, got most up and went over it with a carpet shampoo machine. Turns out that the floor is concrete with a kind of black surface (??), the wetness of the carpet cleaner has lifted the black stuff through the underlay and through the carpet so now have a nice grey circle on the surface.
The stain is about a third into the room and on the walking area between two doors, so can't put hide it by putting furniture on top and a rug isn't going to fit there either. We have a huge rug already in the centre of the room, so there's only a few square metres of carpet showing, is it possible to dye it a darker colour while it's in place? It's a lot of work and money to replace the whole carpet. I know there'll still be a stain, but a dark stain on a dark carpet looks better than a dark stain on a cream carpet.
Any suggestions?