This question comes up a lot and we talk about dyes, india ink, shoe polish, mineral oil, bore oil, boiled linseed oil, etc. Dyes can rub off, india ink is very dark black, and oils need re-applying to look dark and rich. Here's a simple way to do it using a chemical compound that actually oxidizes the wood, causing it to change color. Potassium permanganate is a simple chemical compound of potassium and manganese. Editing to say some folks have brought up potential safety issues with the use of this substance. My use is highly diluted and removes any standing substance from the wood, so I don't believe there is any issue, but of course work safely and take the precautions you would with any potentially flammable substance. It's $19 on Amazon This is a garden-variety new cheap rosewood fingerboard. It's kind of light, kind of purple-ish. Not terrible, but not a great looking board. Take a little potassium permanganate (this is about 1 tsp), it looks like a grey dust... Mix it with water and it turns purple (because of the potassium) - this is about a 7:1 water to powder ratio, though you can alter it to taste. Mix it up and wipe it on to your fingerboard. One note of caution - this stuff is messy and it will oxidize or stain anything porous it comes in contact with. Do this outside and wear gloves if you don't want your fingertips stained. You can do a single application, apply a couple of light coats, adjust the ratio of your mix, etc. Let it dry, rub it out a bit, and boom... It'll look even nicer after oiling.