I didn't say colours exist outside our heads. I said they're produced by the system of light, the surface of objects, and the eye-brain information processor.

Dreams and perceptual illusions are special cases that prove the rule. Presumably, dreams would retrigger memories of colours. Illusions or colour blindness would be due to limitations of the eyes and brain.

But Newton's prism experiment shows that the colour spectrum is contained in white light. So the colours can be split by a prism or by the eye-brain.