Glue seepage, or glue on your fingers that’s transferred to the wood, blocks stain penetration. This usually results in a lighter area. The same can happen with wood putty because it doesn’t accept color the same as wood does. In both cases, you have a lighter area or spot that you want to color in to match the surrounding wood.
Here’s how to do it.
As shown in the accompanying picture, the better method is usually to first seal the wood, then paint in the grain. Connect the grain lines on either side of the lighter area. When this has dried, apply a washcoat (a highly thinned coat of finish) to seal in what your have done, then color in the areas between the grain lines.
When you’re happy with the result (which is rarely an exact match), apply the topcoats.
For the coloring medium, ideally you want to use something that can be removed without disturbing the color underneath if you should want to begin again. The best medium is usually shellac because it can be removed by wiping with denatured alcohol. It won’t lift an oil stain. But it could lighten a water-based stain, so wipe very lightly.
Shellac also dries rapidly, so all steps can be accomplished in a short time.
Universal color pigment, the same as the paint store uses to tint latex paint, can be used with the shellac.
For the washcoats, aerosol sprays are widely available and work well.
Instead of shellac you could use thinned varnish and oil- or Japan-color pigments, but the drying time will be extended to a day or more.