I get intensely vivid 'stygian blue' hypnagogic hallucinations right before I fall asleep. It's an intensely dark/bright blue. Very hard to describe. It usually manifests as a blob of color that pulses from the outer-edges inwards.
Humorously, Negativland, the amazing decades old plunderphonics group, had a whole prank site[1] and a Over the Edge[2] radio show[3] regarding the 'fourth primary color', Squant. This was also the only color with it's own smell as well. Negativland also had a plugin for web-browser that allowed you to 'see' this magical fourth primary color.