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At night they are primarily visible by self-generated light and only secondarily by reflection, and virtually all colors of the spectrum are reported, with a change in color often observed as the UFO accelerates.'
Dr J Allen Hynek - Speech to the American Institute of Aeronautics and Astronautics.
The Emerging Picture of the UFO Problem
As Doc Hynek mentions above in his speech to the A.I.A.A sometimes UFOs have been observed changing colour (possibly due to acceleration) and there's some relevant speculative articles and info below about the subject from folks such as NASA's Paul Hill, the Rand Coporation and researchers Otto Binder and Dimitris Hatzopoulos.
Brightness and color changes are also noted, and while the sample is small (82 cases) NICAP found the following: Of the 25 cases showing a change in brightness, 23 of the changes occurred at the moment of a velocity change (a change of either magnitude or direction). Concerning the change of color, 23 cases showed a color change related to acceleration. While the supporting data are not conclusive, it appears that the spectral shift is to the red upon acceleration.
The RAND Corporation, 'UFOs: What to Do'
All UFO colors stem from energetic, ionizing radiation or radiations, generated by the UFO, which ionize the air.
Of all the visible colors, red and orange correspond to the least energy. They are also the two most common colors associated with UFO low-power operation, such as hovering or low-power maneuvers. The electrons have been given the ionization energy, but not much more, and cascade down in small energy drops corresponding to red or orange. This is statistically probable, as there are more small drops available than big ones.
]UNCONVENTIONAL FLYING OBJECTS, A Scientific Analysis by Paul R. Hill
The glow / luminescence in various colours around the UFO (apparently shape depends on UFO's shape as well as its current maneuver, so the UFO's outline as seen by external observer can change), is thought to be due to ionization of surrounding air (atmosphere around the UFO "lights up", much like what happens in neon-lamps), hereafter referred to as "UFO plasma sheath". Brightness/color changes of the "UFO plasma sheath" seem to be related with thrust/acceleration
Resources about possible UFO physics/ propulsion / technology
Color Changes
Now we come to a different and more spectacular category - the UFO's (still mostly seen in the nighttime) that change colors.Here a remarkable pattern emerges in which the color changes are almost certainly related to the UFO's movements - hovering, speeding up, slowing down, descending, ascending, and high-speed acceleration.
A rough scale would relate the colors to motions in this way:
White - when hovering or moving at uniform speed.
Red and orange - when accelerating.
Blue or blue-white - at extremely high speeds.
Green - when making right-angle turns or performing other aerial maneuvers.
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