All wood exposed to sunlight changes color over time. In furniture, this acquired color is called “patina” and is desirable. Anything organic exposed to the sun changes color, including you and me. Some manufacturers use a finish additive that acts like a sunscreen, which delays (but does not prevent) the natural color change.
In some woods truly spectacular things happen when wood oxidizes. Curupay, for example, is a tropical wood that tans like no other.
Although Curupay is an over-achiever in this department, all woods, like other living things, will build character with age. This is one of the many reasons we have such a strong sense of wood being “warm” or “real” in a way that few other materials, natural or synthetic, can compare to.