Reaction Staining
Instead of adding a color to furniture, you can enhance the wood's own color through chemical reaction.
Ammonia fumes can stain new wood to look 100 years old, and in a matter of days. On small areas, ammonia can be brushed on directly. The ammonia reacts with the tannins in the wood.
Strong tea can be brushed onto light wood, with little tannin, to darken it with its own tannins. This can be darkened even further by following with an application of ammonia.
Oak, walnut and mahogany will get browner with ammonia and blackish brown from rusty nails. Pine, maple and birch will get browner with tea or tea followed by ammonia. Some stains will darken as they dry, so go a step at a time. Wood will also look even darker after a finish (lacquer, varnish, tung oil, or shellac) is applied.