The wood in the table image MrYorke has provided is neither hard maple or sycamore (Acer species), the common base materials for creating harewood, aka greywood. I think the table is an oak of some sort - the coarse grain and splitting suggests it, and this coarseness indicates it's not any of the maples (Acers). Marcros has kindly mentioned my writings on creating harewood, and provided a link to one of my harewood pieces. Ferrous sulphate is the chemical to turn a maple into harewood, as others have said, but it also works on oak to turn it grey, as it does on other tannin rich species, although perhaps not so attractively, all depending on your taste. I'm not sure why it works on maples because I can't recall any of them being known for their tannin content.
The grey in MrYorke's image isn't really like the grey developed from exposure to light and air, so I suggest a chemical approach using oak and ferrous sulphate, or perhaps using oak or another species, and experiment with the dye from AG Woodcare Peter suggested. I've never used the AG Woodcare dye so I can't comment on its effectiveness. One thing to watch for, whichever way you go, is variation in tone and shade from one board to the next in a panel depending upon how the wood either takes up the dye, or reacts with the ferrous sulphate. A consistent colour throughout may not be achievable without judicious use of toning coats, and if the client expects consistent, not variable, this is something that will need to be addressed and resolved. Slainte.
