Wings show emotions. The more brilliant the color and sheen the more love and happiness is in and has been in the persons life. The duller the color and sheen the more hatred and sadness is and has been in the person's life. When someone is born their wings are just fluffed feathers. They grown into adult wings around the ages of 10 to 16. When the adult feathers grow in the brighter or duller the color of the wings it is said to show the amount of love or sadness that will be in the persons life. A prediction of the emotion in their future of sorts.
All this has been true for Sherlock Holmes as well. Everything except the prediction of emotion of his life. Sherlock's wings, when they grew in, where the vibrant black of a raven. Showing years of love. Sherlock never thought this would be true. Already his life was full of hatred and sadness. His older brother is a prodigy. Going to university while his wings were still growing in. Sherlock was always living in his shadow. Sherlock's father's wings were the dull grey of an old falcon, his life had been full of sadness and hatred and he takes it out on his youngest son. Sherlock's mother, with wings of a once brilliant colours of a swallow, is dying of a cancerous tumor in her wings. The only way to rid her of it is to remove them, which she refuses to do. Yet Sherlock's wings are a vibrant black. And as his wings grow in they bring the promise of unconditional love and happiness. But Sherlock refuses to hope for what he believes to be false prophecy.
All this about wings was true of John Watson as well. And when his wings grew in he was ecstatic. The bright brown of a hawk's wings shimmered in the sunlight when he flexed them. The showed him that his life would be forever filled with love, more of the same from his childhood. His mother and father loved each other dearly and although he fought with his older sister sometimes, they always made up. He didn't know how right that prophecy would turn out to be, or how wrong.