Yellow and green are the colors of vegetation, and the difference between them (for example with ripe and unripe fruit) has practical consequences that one might want to talk about. Yellow dyes also happen to be relatively easy to make. The cultural significance of blue, on the other hand, is very limited. As noted earlier, blue is extremely rare as a color of materials in nature, and blue dyes are exceedingly difficult to produce. People in simple cultures might spend a lifetime without seeing objects that are truly blue. Of course, blue is the color of the sky (and, for some of us, the sea).
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