They are not just an erotic authors imagination. Those glorious flashes of ecstatic color actually occur for a select group of lovers.
An article in today's Scientific American's on line edition declares;
"Scientists are just beginning to document sexual synesthesia, a phenomenon that can bring consciousness altering washes of color and fantastical scenes at the moment of orgasm"
For those of you who wish to dig into the nitty-gritty of the phenomena, here is a link.
https://www.scientificamerican.com/article/sexual-synesthesia-paints-the-world-in-color-at-the-moment-of-orgasm/
Gary