Most species of wood will react to a potassium permanganate solution by turning various shades of brown. This method is similar to using lye on cherry, but doesn't require neutralization. You can add tannins to any wood by drenching it with a strong tea solution. After the solution dries off thoroughly, wipe it down with a dilute iron acetate solution. This will create some wonderful colors, especially in pine.
Colors range from silver gray to dark black-brown, depending on the concentration of the tannins.
Old Growth has several stains and 'aging' agents for wood. There is a mineral solution followed by a catalyst solution. This system works well for creating different effects and behaves somewhat differently with each species of wood. It also creates color several millimeters deep into the wood, which most other treatments and finishes do not accomplish. This means you can still sand somewhat aggressively after coloring with this product.