How rainbows with NO COLOR are possible

Pilot glories: I am a retired airline pilot - and a former hang glider pilot. The "pilot glories" I have observed for almost 50 years are not so much about a rainbow forming around the shadow of a flying aircraft as the fact that the light is more intense immediately around the shadow itself than the surrounding area. My personal theory was that since any heavier-than-air aircraft stays aloft by forcing a considerable amount of air downwards when passing through it, the momentary pressure differential in the air caused the air to act like a weak lens, concentrating the light around any shadow formed by an aircraft. This phenomenon is easily seen when a shadow is observed on clouds, snow or even water and the ground. What I always found very interesting is the fact that this effect is very similar whether you are observing the brightness around the shadow formed by a 200 ton jet you are flying in at 500 knots or a 400kg ultralight at 80 knots. I have even observed this phenomenon when flying a hang glider - total weight a mere 120kg and very low speed! For some reason the speed or weight of the aircraft does not appear to be a determining factor...
Riddle me this, Physics Girl!