Because the color gray is between black and white — actually a mix of the two — it is often used to denote something dull or monotonous. When the sun comes out after a long stretch of gray days, you suddenly feel alive again. Everybody to the beach!
You might feel old when you see your gray hairs, but I think they make you look wise and distinguished. Because gray is dull and indistinct, you can refer to something not quite clear as being "in a gray area." You often find some gray area between right and wrong . . . especially when you are accused of doing something wrong. Don’t worry about the British spelling "grey": it’s interchangeable with the American English spelling.