You can try to find some pure lemon oil WITHOUT any petroleum, silicones etc in it, but the best stuff for your board is Fret Doctor, you can order it here: Fret Doctor
A small bottle will last for years.
I know a guy who darkened up his MIM tele neck with brown shoe polish. It went from being very white to a nice yellowish brown that you associate with maple guitars. It actually stains the finish and does not leave any trace of residue oned buffed off by hand. It would likely have the same darkening effect on a finger board. I have darkened up fretboards on a couple of accoustics with dark scratch remover that I bought in home hardware. It did a great job and did not darken it nearly as much as the colour of the scratch remover would make you think. Just wipe it on & wipe it off. Repeat until it is the colour you desire.
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