https://www.cbc.ca/news/canada/british-columbia/pink-purple-fog-kelowna-penticton-1.7100456
To summarize: A foggy condition that gets it's coloring from a sunrise or sunset.
The rarity is obviously because the fog essentially has to be localized (so there is a clear sky behind it to provide the coloring), and rather light (to let the light bleed through).
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