Burn-in lacquer sticks. If you can't go that way, they sell colored
wax crayons at the home centers. Pick a close match (buy a selection
and return what you don't use) and rub it in. Scrape off the excess
with a credit card (then send the card to me - you won't want it anyway
as it'll be a little dirty). They also sell felt tip finish pens that
have stain and a polyurethane(?) finish in them. You can top coat with
one of them.
If there are other doors the same size you may want to swap the damaged
one with another in a less conspicuous location.
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