The cover sticks are available at Lowes or Home Depot. I've found you’ll need to select a couple as exact matching is rare. Pick one you think will match, and one darker or lighter. They are cheap and you may have to do some blending. You could take a small door in to color match. Cannot tell you what color to select as there aren't color standards for these. One walnut may not match another manufacturers.

Thanks! So a waxed-based one, rather than a marker?

I like the wax version for bigger boo-boos.