When did differentiating colors become important? (Why were color names so often conflated in long-ago times?)

You should check out the article "Basic Color Terms" by Berlin and Kay. They're linguistic anthropologists, so this question is less a history question and more an anthropology one. What they found is that there's a basic sequence according to which cultures adopt color terms. Everyone can see all of the colors, of course, but the assigning of a color term reflects the necessity to make that distinction. What Berlin and Kay did was show an array of colors to people whose native language was not English. I believe they had over 100 different language represented. What they found was that some languages didn't have any real color terms. They just divided things into light and dark, which B&K call WHITE and BLACK. In other words, they'd show them all the colors, and the people would divide them into those two categories, and if asked what specific color swatch represented the quintessence of each term, it would be the white swatch for the light colors and the black swatch for the dark colors. Then, invariably, if they had another color term, it would be red. That term would cover all the reddish colors in the spectrum shown. By the way, they didn't consider something to be a color term if it was merely a reference to an object of a certain color. So "peach" isn't a color term because it was a specific object, rather than being an independent term for a color, irrespective of an object that was that color. Anyway, after red, cultures would add I believe blue. Then after blue, it was either yellow or green (whichever they didn't add in the earlier step). Then the next color added would be green or yellow, so that now they'd have white, black, red, blue, yellow and green. Once they had all of those terms, they'd add a bunch of other terms in no particular order, but those terms would have priority--pink, brown, orange, gray, and purple, I believe. After that, they might add an infinite variety of different color terms. That's why you'd have a culture with terms like green and yellow merged. They didn't need to differentiate those colors so importantly that they needed 2 words, at least initially. But Berlin and Kay would show the color spectrum to people who spoke a language that had a term like that--that referred to yellow and green. They'd ask which color was the quintessence of the meaning of that term. Sometimes people would choose green, for example, meaning that although green and yellow were included, green was the main reference point and yellow was kind of a form of green, much like for us today, turquoise might be a form of blue, rather than its own color, to some people.