| Unless you are fortune enough to have been given some of the varnish that was applied by the maker when the instrument was made, re-touching (actually varnish matching) is, IMO, one of the most difficult jobs and most commonly screwed up jobs in amateur instrument repairing. Modern sprayed on synthetic finishes that are usually used on mass produced basses are especially hard to match. Bottled oil and spirit color varnishes sold by violin supply houses when used for touch-up usually end up looking far worse than if you had done nothing. They are fine for sealing the wood on worn edges, but for places that are highly visible, think twice before you do anything.
My advice is to either (1) take the instrument to someone who is known to be good at varnish matching for the touch-up, (2) apply one thin coat of a clear spirit varnish on just the bare wood to keep dirt and other contaminants from getting into the wood so someone will be able to do the job properly at a later date or (3) do nothing.
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95% Retired Mid-Western Luthier
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Large varnish chip! 
