And of course the white finish issue is equally applicable to powered aircraft constructed of the same materials. Anyone seen a black Europa, Jabiru, Sky Arrow, Polaris, Grob..........?
The main point at which it is difficult to see a glider is when it's pointing at you or away from you at the same height because of their low frontal area. However gliders tend not to be doing that for long so a visual scan that looks at the same piece of sky twice with a few seconds between is very likely to pick it up as it turns. If it wan't easily visible on the first sweep it is probably a lot easier to see on the second.
Mike