There are a few ways that wood ages when exposed to the elements and light. How the wood looks as it ages is dependent on a number of factors: type of elements, amount and type of light, where the wood is harvested from (i.e., closer to the bark or heartwood), how it is finished, etc. All of these and more will affect the look of the material over time.
And, of course, the type of wood species can be a large factor. Obviously, since not all woods look and behave the same when growing, cut, and machined, similarly not all wood looks the same as it ages.
In most cases, the sort of aging that affects colour is oxidization, and in many cases the typical result is lightening or greying. But some species have a chemical composition that result in more obvious colour changes as it reacts with the environment. In general, UV light accelerates oxidation by interacting with the chemical bonds that hold molecules together. UV light also interacts with aromatics in material. As the material breaks down and reacts with oxygen (and the oxides further interact and degrade) the bonds between the molecules that absorb and reflect spectra break and reform into different molecules. The light that reflects from these new molecules results in what humans perceive as colour.
Some varieties of cedars turn almost silver over time. Poplar or American whitewood turns greenish. As you point out, purpleheart ages to a spectacular purple. (As an aside, the colour blue in nature is very interesting -- often what we perceive as blue isn't a result of pigments, but white light interacting with the physical world such that we are seeing a partial spectrum of light. Moreover, blue pigments tend to be a bit unstable and fade quickly.)
As you point out, in general most wood just loses colour. But if you look carefully, even mundane wood from different species and different parts of the plant is different enough. A stand of dead pines look totally different than a stand of flooded oaks or elms. And standing timber, once made into lumber or shakes, age in their own unique way even though we'd probably describe the colour as "grey".
