You make a good point regarding the author's understanding of color blindness, however gray-scale vision absolutely does exist in humans, albeit in a much small

You make a good point regarding the author's understanding of color blindness, however gray-scale vision absolutely does exist in humans, albeit in a much smaller proportion of the population than common red-green colorblindness: https://en.wikipedia.org/wiki/Achromatopsia https://en.wikipedia.org/wiki/Achromatopsia