Simon, interesting and new to me. I really don`t understand the reason for white sky periscope. Mostly the colours of the top was various shades of grey, like on the picture below, I think most of the colours were darker than on this picture though, but I believe it depended very much upon in which aereas you were operating.. We carried out some tests and figured out that dull dark grey, almost black with a shade of dark green was the optimal for the water we were operating. But I guess it was the wake of the pericope which probably was the worst problem.
Tore