They don’t.
Some things are well known because they still exist. They know coca-cola is probably red, military uniforms are olive green. Maybe that particular skirt wasn’t this pastel color but some of them were. You can look find them in museums or in books with color photographs from later.
But other than that they mostly just guess. Maybe that’s the wrong word, they choose. They get some info from the greyscale but at a certain point they just pick and go. Modern digital tools mean that they can even change colors at any point in the process and just make it look good.
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