The Royal Air Force did a study which concluded that the best
colour to provide the best chance of being seen was black, which is
why all RAF training aircraft, except the GRP ones, are painted
black. From my limited experience in seeing these aircraft in the air
they certainly seem to be more easily picked out from most
backgrounds.

Canadian Air Force trainers are also painted black. Best contrast against sky, clouds, snow, and prairie. Unfortunately, not possible for most gliders, and rather warm to the touch for ground handling.