Many nebulas are red (Halpha) like Eta Carinae, M42, NGC 7000, Tarantula, California nebula, etc.
I always thought the nebulas emit light from themselves (from what energy source ?).
In
Wikipedia I read that Xi Persei a hot O star lights up the California Nebula NGC1499, but how can it be red when lit by a star of 37000 K (bluish) ? Another example, the Witchhead just east of Rigel is whitish and it lit by the bluish Rigel which indeed is the right color.
Does somebody know this why NGC1499 is red and not bluish white ?