From The Economist:
Languages divide the spectrum up in different ways. Welsh speakers use “gwyrdd” (pronounced “goo-irrrth”) as a general word for green. Yet “grass” literally translates as “blue straw”. That is because the Welsh word for blue (“glas”) can accommodate all shades of green. English-speaking anthropologists affectionately squish “green” and “blue” together to call Welsh an example of a “grue” language. A few of them think grue languages are spoken by societies that live up mountains or near the equator because ultraviolet radiation, which is stronger in such places, causes a progressive yellowing of the lens. This, the theory goes, makes the eye less sensitive to short wavelengths (those that correspond to the green and blue parts of the spectrum). Unfortunately, though the Welsh do live in a hilly country, it is hardly mountainous enough—let alone sunny enough—to qualify.
The ultraviolet theory, however, is just one idea among many in the debate about the psychology of colour.