Is the color inside the Trex fence material the same as on the surface?

Trex adds color during the manufacturing process so it exists inside the material as well as outside. The color (pigment) is added early on in the manufacturing process while the plastic and wood are being mixed together and heated. The same concentration of pigment exists in the core of the board as there is on the outside. 

The outer surface does appear to be darker initially than the inside of the material. This is due to the size of the wood fibers, as the material goes through the die during extrusion, most of the larger fibers are forced deeper into the core of the board, with only a small part of it exposed to the surface. If you sand away more of the outer surface, which is mostly plastic and very fine wood fibers, more of the larger wood fibers will be exposed. Those larger fibers don’t accept as much pigment, so it will look lighter. As the fence weathers, the exterior becomes lighter and that is why exposed inner material more closely matches the outer surface over time. In the case where your fence has been affected by abrasions during installation, scrapes and dents will become much less visible after a season.

You can learn more about Trex Fencing's durability here.