"A Faerie’s wings are based off their elemental abilities - should a Faerie excel in the element of Earth and a little bit of water their wings are bound to be an impressive colour of green with a hint of blue. The level of power also affects the size of their wings. Although unseen to the human eye powerful Faeries learn to conceal their true wings so as not to draw attention. "
I think just their general appearance, like how his/her powers have shaped them.