Pour coffee on it
Make a small cup of very strong coffee and apply it to the scratch with a soft cloth. This should stain the exposed wood to the darker color of the finished wood.
Color it in
Hide the scratch by rubbing it with a little shoe polish of a matching color. Alternatively, try an eyebrow pencil—soft, oil-based, and brown colored—ideal for camouflaging anything from small surface scratches to deeper ones.