The existence of several meanings in a single word. Such a word is polysemous.
Some colours are harder than others to describe.
Examples are words in ancient Greek and Latin that denoted a range of hues, or even just dark ones and light ones, rather than specific colours. ‘Thus kyaneos—from which is derived the modern…cyan—almost always designates a dark color, but it might as easily be dark blue as purple, black, or brown. Thus again glaukos, which the archaic poets used extensively, can sometimes express green, sometimes gray or blue, sometimes even yellow or brown. It conveys the idea of a color’s paleness or weak concentration rather than a precisely defined shade. That is why it is used by Homer to name the color of water as well as the color of eyes, leaves, or honey’. —from Green: The History of a Color, by Michel Pastoreau (p. 15).
And don’t forget that instead of calling the sea a colour that we might recognize, Homer famously calls it ‘the wine-dark sea’. Which is to say: dark or opaque as wine could be—not actually the hue of wine! (Though possibly wine in his day was considerably lighter than your average Pinot Noir or Californian Cabernet Sauvignon, I do not know.)
