Dunno - but I'll try and think it through ...
We all know the sky turns red in the West in the evening when the weather is set fair for the following day and if the Eastern sky is red in the morning it looks bad for the coming weather.
It's to do with refraction of the light so that we are seeing the red end of a specrum when the sky is red - high pressure in the West or damp air in the East.
If the sky is red in the early evening it seems to be neither of these two - but some atmospheric condition or other is showing you a sky lit by the red end of the rainbow.
Guess you knew that