Mine is the colour of the back of colourbond.
No lining or paint.
Can't see how it matters really.
It is useful to have lighter coloured wall if you wan't to make flat frames by aiming the scope at the wall. I have a piece of gyprock on 1 wall that the scope can aim at easily. I then use an old flash and aim it at the back wall. This reflects around nicely and gives me good flat frames.