I’m not sure it’s what you mean, but if you are talking about something I’ve heard called ‘the void’ I have experienced this. The moving colours are a mix of things I believe, the ones you see when first closing your eyes are usually essentially static that is caused by random firing of neurons in your eyes. Later they are basically a form of HI. I too get this. The ‘void’ experience was one that I encountered when trying WILD once. I was in an extremely relaxed state and suddenly I felt disconnected from my body, life and everything. It was as if I was floating in nothingness alone. The blackness I saw then was blacker than even when eyes are closed and covered in a dark room.
I personally found the experience extremely relaxing and would love to repeat it. I have spoken to others though who found it to be disturbing and felt incredibly lonely.