Re: 90,000 feet
There are sound engineering reasons for gliders being (mostly) white. Most modern gliders are made of glass fibre or composites, and white is the most reflective colour that they can be - any other colour absorbs wavelengths which might well damage the aircraft. Metal gliders, like the Blanik or Pilatus B4, or wood and fabric gliders, like the Ka6, can be any colour you like.
Wood and composite gliders aren’t very reflective to RADAR - and even the metal ones tend to have a small cross section from most angles.
And as for radio - what good is that? We do have it, and we use it when necessary (check us out - broadcasting to you on 130.1) - but it doesn’t give you anything other than a voice. You won’t get range, altitude, direction or anything else from a radio.
On the other hand, the best pilots (whether commercial, military or private) I know have all been glider pilots at some point. The biggest blowhards pontificating on GA haven’t even sat in a sailplane.
Oh, and FTR, in the UK glider pilots use feet and knots - same as every other pilot.
My personal record, in wave, was 9.5k ft, in a blue L13 Blanik (which, sadly, was wrapped around a tree in a storm in 1990. Luckily, no one was in it at the time, or hurt - but a sad end to absolutely my favourite glider ever, and the one that I solo’d in)