That's assuming the only purpose of color vision is to see the colors of other living organisms.
There's color in nature beyond life, such as in minerals and other chemicals. There's also color in life that isn't necessarily meant to convey something —such as the green of plants or the red in blood— that could be useful for finding food, for example. Interestingly, hemoglobin seems to have come to be > 400 mya too [1].
Moreover, color can help with contrast in vision. Two materials could reflect the same amount of light, but in different wavelengths.
There's color in nature beyond life, such as in minerals and other chemicals. There's also color in life that isn't necessarily meant to convey something —such as the green of plants or the red in blood— that could be useful for finding food, for example. Interestingly, hemoglobin seems to have come to be > 400 mya too [1].
Moreover, color can help with contrast in vision. Two materials could reflect the same amount of light, but in different wavelengths.
[1]: https://www.ox.ac.uk/news/science-blog/ancient-blood-lines-t...