Azek plastic is becoming a very popular alternative to wood on many home improvement jobs, repair jobs, and new construction. The reason--it lasts longer than wood. Since it is plastic it does absorb water and thus does not rot, which is often a major problem with wood. This is especially true with north facing walls of houses. Almost every time that I see wood on a home that should be replaced before I paint I recommend to the Home Owner that they use Azek.
However, If you choose to have your contractor use it on your next project, and you intend to paint it, then you must consider that any paint that goes out it must be 100% acrylic and have a Light Reflective Value (LRV) of 55 or above. This means that most dark colors are out. Usually LRV can be found on paint fans. I am assuming that the reason for this is that if the paint absorbs too much light it will lead to the paint heating up, which will then lead to paint failure.
Azec, which is white, does not have to be painted. However, it is the only way to cover the nail holes and make the edges of the Azek that has been cut look nice.
So, if the trim on your home is painted black or brown and you decide to use Azek for a repair of rotten wood, you will have to change the color of the rest of the trim to make it match the new lighter color that will be needed to paint the Azek.