Its all to do with the visible light spectrum, lilac has the shortest and wavelength whereas red has the longest wavelength. As fog is water droplets in the air some of the light produced is reflected away but due to red light having a longer wavelength more actually reaches the ground. White light is a mix of all waves of light whereas SOX/SON have a higher percentage of red/orange light in them. This is why a move away from SOX/SON would reduce sky glow as there would be less long range red light waves to be reflected upwards.
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Perhaps they could fit colour changing LEDs that could use white light for normal driving conditions then change to the orange glow of SOX during poor visibility.