How do I repair splits in a wood cabinet going with the grain?
Colored wax is very minimally disturbing (to the existing finish) and easily reversible, should you want to redo the repair.  Buff it with soft cloth to match sheens and you can even overcoat the whole piece with a good paste wax, afterwards.

For wood grains that have a range of colors, I like to blend (rub in) several colors along the crack.  That way, the repair is less monochromatic and therefore more natural looking.