You want to highlight the high points. So, paint the cover in olive, say. Allow to dry, then take the same olive & lighten it a bit, light grey may be better than white.
Now drybrush some of this onto the cover, just a little.
Let that dry, make up some even lighter olive and just do the parts of the cover on the frames.
This will give it some depth & create an illusion of wear &, I suppose light distribution is the right term.
Try this, wear some baggy trousers, sit outside with one leg over the other.
The trousers may be the same colour all over, but in the folds will be darker, the tops of any folds will be lighter.
Comprendez?
Good Luck
Pete