One of the great challenges of adding to someone else’s wood work is matching their stain. Often the previous woodwork has aged and changed color. In addition it might have been stained, dyed, or glazed.
So how do you figure this out?

First, here are some basics:

1. A stain changes the color of a wood by darkening it and adding color to it. Usually, the color has features of another natural wood. A dye tints wood — it might yellow it or redden it — but it only adds a shade. This is useful in trying to adjust a stain color. A glaze goes on after the finish. This can give it other qualities since it doesn't cover with the same uniformity of a stain. It can fill wells and corners more.

2. Wood changes color over time. Not all woods change colors the same amount. Cherry changes huge amounts as do walnuts — the more direct sunlight the faster. The impact of this is that your cherry from last year won't match a piece that was kept in a closet or a new door made by the exact same manufacturer. Over time however, the newer piece will approach and perhaps completely catch up with the older piece.

3. Woods of the same variety can have vastly different looks. This is because no two trees are the same yet alone trees grown in different climates or different conditions like a sloping hill vs. moist flat land. The wood within the same tree has different colors. The core of the tree is darker than the outer rings. This is why there are many varieties of veneers which account for these different locations wood is taken from within the tree. For high-end projects, veneers have to be ordered to line up in the same way they were in the original.

4. The type of wood finish effects how a wood will color over time. This is important because if a new piece of wood work is added to existing woodwork and the finish is different it may begin to look different over time rather than more similar.

To determine whether stain or dye has been used, take apart a piece of the wood work and look at it from an edge where a finish surface meets an unfinished surface. Here you'll be able to see evidence of staining and dying because the finish, stain and dye run all over the edge but they run over differently. In other words, if there was staining you'll see stain without finish on it and you'll see finish without stain underneath it.
It also helps to look at wood that has been kept in the dark. If there's no evidence of staining yet this wood looks decidedly different than the finished wood, you can count on the fact that sunlight has made the difference. More clues can be gathered by looking at how the finish was applied. Brush application is typical of painters and painters typically finish with varnish vs. polyurethane or other higher quality finishes.

Once the examination is done, it's best to create samples for the customer. Sometimes the aging can be matched very closely through a combination of dyes and stains. This then can be protected by a UV blocking finish. Still, there are no guarantees that the UV protection in the finish will last forever or that the other wood won't continue to age differently than your well matched new section.

The only saving grace is that wood has a great amount of variability and over time it makes the problem disappear even if you can see it when you look for it.