Why does my bass (maple fretboard) turns darker and darker as the days past... How can i prevent it?

Answer 1

Here s a good article on fretboard care. Following the recommendations should reduce the darkening of the wood. The normal use over time however will darken the wood from contact with the oils found in skin.

Answer 2

If it is gloss finished, it shouldn't really change color, but if it's a matte finish (especially if it's tung oiled) the wood absorbs the oil from your hands, as well as the residue from the chemical reaction of the strings corroding. If you want to avoid it, I'd suggest re-oiling it every few months.

Answer 3

It's normal. You might try washing your hands before you play.