About 10 years ago when I asked a UK agent/ repairer for one of the major German companies about this I was told that they used thin gelcoat with less white pigment added before spraying with PU on their new gliders. The less pigment the better the polymerisation during curing I was told and colour density wasn't important as it was covered with PU. The gelcoat still looked completely white to me where I could see it under chips and scratches but that's what I was told.