How do you describe the color purple?

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How do you know if the colors you see are truly the colors everyone else sees? By definition, colors are the perceptions of an individual's perspective of a light's differing qualities. How do you know to call something purple? Because you were taught that a certain color was called purple. But what if another person was taught it was called green? Who would be right? What about if your friend was colorblind or completely blind? How do you describe a color without making a reference to another color? How do you describe the color purple?