This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Ophthalmology
Which of the following is an example of extraspectral colour? 
Select and explain. 
A. Red, B. Green, C. Blue, D. Purple
D: Purple
Answer: D: Purple 
Explanation: Another observation of basic impoance is the demonstration that the sensation of white, any spectral color, and even the extraspectral color, purple, can be produced by mixing various propoions of red light (wavelength 723-647 nm), green light (575-492 nm), and blue light (492-450 nm). Red, green, and blue are therefore called the primary colors. Ref: Ganong's Review of Medical Physiology 23rd edition, Chapter 12.