The thing that makes denim quite special and the thing that makes denim fade so well is the fact that it consists of a warp and weft of a different colour.
The construction is pretty simple. The "warp" - in denim - is the yarn that runs parallel to the selvage. This is the yarn that is vat-dyed blue and this is what gives denim the blue appearance. The "weft" runs from selvage to selvage at right angles to the warp yarn. The weft is also what gives denim a noticible different colour on the back side, which is often cotton yarns of a natural, un-dyed colour. The weft is also known as the filling or the filling yarn. It should also be said that the weft may sometimes be vat-dyed, like the warp, and this creates a fabric with a very dark blue appearance and some spectacular fading abilities.
And you can find denim with a weft that is of a different colour than blue - like in this instance, purple.