"... and it seemed no two people quite agreed as to what indigo was, and I thought I would like to have an experience of indigo. And I built up a sort of pharmacological launchpad with amphetamines and LSD, and a little cannabis on top of that, and when I was really stoned I said, 'I want to see indigo now.' And as if thrown by a paintbrush, a huge pear-shaped blob of the purest indigo appeared on the wall. Again it had this luminous, numinous quality; I leaped toward it in a sort of ecstasy. I thought, 'This is the color of heaven.' ... I thought maybe this is not a color which actually exists on the Earth, or maybe it used to exist or no longer exists. All this went through my mind in 4 or 5 seconds, and then the blob disappeared, giving me a strong sense of loss and heartbrokenness, and I was haunted a little bit when I came down, wondering whether indigo did exist in the real world."
Oliver Sacks, in his new book "Hallucinations," discussed in a nice interview on NPR today.