When shell is folded repeatedly (crease area), it becomes particularly porous. Dirt and residual polish will get trapped. Hence the darkening. Thats the nature of shell. There is no real effective way to totally remove the impregnated dirt/polish.You may try using adhesive tape to lift off some of the dirt, followed by a light layer of Reno and vigurous brushing to smooth out the surface.
8/26/15 at 3:28pm