I can't give you a detailed plot summary without knowing which 'All the Colors of the Dark' you're talking about, it's a surprisingly common title! There's a 1972 giallo film by Sergio Martino, and a 2024 fantasy novel by Chris Whitaker. They are completely different beasts.
If you mean the film, it's a wild, psychedelic Italian thriller from the 70s. A woman named Jane, traumatized by a recent miscarriage and a carjacking, starts having nightmares about a man with a strange eye. Her therapist suggests an... unconventional cure involving a local Satanic cult, which, predictably, makes everything infinitely worse. It spirals into a paranoia-fueled nightmare with black masses, ritualistic murders, and a twisty plot about doppelgängers. The ending is famously ambiguous and unsettling, leaving you wondering how much was real and how much was in her shattered psyche.
For the novel, it's a whole other story—a sprawling, decades-spanning tale set in a small Missouri town. It follows two kids, Joseph 'Patch' and Misty, who witness a terrible crime in 1975. The story jumps forward to 1990 where Patch, now an adult, is trying to protect a young girl named Stacey from a notorious serial killer he believes has returned. It's a much more character-driven, melancholic saga about trauma, friendship, and the long shadows cast by violence, with a very different kind of atmospheric dread compared to the psychedelic horror of the film. So yeah, you gotta specify!
Titles are a minefield sometimes.