No, there are no truly unknown colors in the sense of colors that physically exist but are undetectable to humans. However, the perception of color is complex, leading to some interesting nuances.
Understanding Color Perception
The colors we see are a result of our brain interpreting the wavelengths of light that reach our eyes. We have three types of cone cells in our retinas (short, medium, and long wavelength sensitive), and the combination of signals from these cells determines the color we perceive. As stated in one reference, "The colours we see are an artifact of the brain and consciousness." [Quora]
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Physically Possible, Perceptually Impossible: While any wavelength of light exists physically, it's the combination of signals that our cone cells send to the brain that dictates our color perception. Therefore, certain combinations of light frequencies might exist that our eyes cannot translate into a uniquely perceivable color. This is partially explained by the overlap in sensitivity of our cone cells: "The spectral sensitivity curve of medium-wavelength (“M”) cone cells overlaps those of short-wavelength (“S”) and long-wavelength (“L”) cone cells. Light of any wavelength that interacts with M cones also interacts with S or L cones, or both, to some extent." [Reference Text] This means there are limitations in the range of colours we can distinguish.
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Impossible Colors: Research has shown that some color combinations, like reddish-green or yellowish-blue, are theoretically possible but cannot be perceived by the human eye under normal conditions. [Livescience] These are sometimes termed "impossible colors." This does not mean there's an unknown color, but rather a limitation in our visual system. The reference explains that "forbidden colors — reddish green and yellowish blue — are real," but only accessible through specific experimental manipulations.
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Errors Related to Color Representation: Many references highlight errors encountered in software and programming due to incorrect or undefined color codes. Examples include error messages like "unknown color name" in programming languages (Python, for example) [PySimpleGUI], or undefined color tags in logging libraries [Loguru]. These situations don't represent unknown colors themselves, but rather errors in the code specifying or representing known colors.
Conclusion
The concept of an "unknown color" depends on how you define "color". There are no physically existing colors imperceptible to humans. However, our visual system's limitations prevent us from seeing certain combinations of wavelengths, and there are technical issues in coding where color codes might be wrongly defined.
