Qualia
The question hinges on whether color is a product of the mind or an inherent property of objects. Whilst most philosophers will agree that color assignment corresponds to spectra of light frequencies, it is not at all clear whether the particular psychological phenomena of color are imposed on these visual signals by the mind, or whether such qualia are somehow naturally associated with their noumena. Another way to look at this question is to assume two people ("Fred" and "George" for the sake of convenience) see colors differently. That is, when Fred sees the sky, his mind interprets this…
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Source: List of philosophical problems (CC BY-SA 4.0)