Mahogany or dark walnut can look well against a light background, but neither mixes very well with pine or any other light grain. So if your built-in must be given a natural (transparent) finish, it should be made fairly dark so it won't clash with the mahogany furniture.
In rooms furnished in mahogany you can, on the other hand, use a plywood with a mahogany facing if you wish. In fact, you can get plywood faced with a large variety of better woods such as birch, maple, walnut or even korina, and color them as you wish.
The finish that you can apply to any built-in piece will depend upon the material from which it is made. For wood surfaces (including plywood) you can use either transparent or opaque finishes. The transparent finishes consist of shellac or varnish, oil-, water- or spiritstains. The opaques are water and oil paints and enamels. However, there are a number of new finishes available that are well worth trying if you want something both attractive and different.
Any wood that has a hard grain with softer wood between it — such as fir — needs something to prevent the grain ridges showing through an opaque finish. In using stain on such a wood it is necessary also to equalize the absorption to achieve even coloring and a rich finish. A very satisfactory material for this purpose is Firzite. This is available in two types. One solution is clear so that you can use it under stains as a wood sealer. The other is white. It will serve as an undercoat for painted surfaces, in addition to its primary function, and form the base for blond effects that are growing in popularity. White Firzite can be tinted with color.
Not all of the so-called blond finishes need actually be blond. Both the clear and the color finish can be applied in such a manner that the wood grain shows through. A number of paint manufacturers are producing excellent finishes that will give this effect — either by putting on and rubbing off (glazing), or applying in a thinned state. Sapolin Paints, Inc., for example, puts out a series of enamels in color that dry extremely hard and have a hand-rubbed appearance.
If you mix some of this enamel with twice as much turpentine you get the wood-grain effect. This thinned enamel is applied to the smoothly sanded wood. When that coat is dry it is covered with a coat of the "natural" enamel that has no color in it.
Then there is the N-B paint process that lets you make your own wood grain on any surface. All grease has to be removed from the surface by washing with strong Oakite or ammonia solution in warm water. This is washed off and the surface finely sanded. A coat of N-B ground color is applied, and two days later a second coat is applied which is later sanded. Over this is brushed a thin coat of the N-B graining compound.