This is a good point.
As far as I know, in previous competitions the walls have been either white, wooden, or clear acrylic (especially if you look at Rescue A videos before 2015). As Naomi says there is no mention of the colour, but it should not be the ball's colour for good reason, and I would say it is unlikely for the wall to be black or silver.
A difficult option might be clear acrylic. We will discuss on this matter with the TC.