He knows he should tell John when he starts to lose his vision. There might be a cure, some drug he can take to learn how to see cerulean and puce and pomegranate again. Perhaps John knows the name of some fantastic ophthalmologist who can take one look at him and know what's wrong, why he can't distinguish between blue and green anymore or why the world has suddenly become so dark, so bland. But his pride won't let him.
Each time he tries, the words fail in his mouth, die in infancy and refuse to be born just right. He seems to be changing the subject a lot. Pauses between sentences, a sudden turn of phrase. Surely John might realize something was wrong by now, by the way his sentences skitter to a stop, seem to jump from morose to cheerful just in time. But he'll be damned if he's going to lead him along that dark path, if he's going to tell him just why he seems to be spending more time staring into nothingness, dreaming up names for colors he can no longer see.
It's at a crime scene – naturally – that John begins to realize what is wrong. It's becoming harder for Sherlock to fake that he can tell what brand of jeans the victim is wearing by texture alone, or that he notices Lestrade's wearing a red shirt and not a purple one. The final straw comes when Sherlock falls flat on his face because he couldn't see the shift in color from street to curb. It's sheer luck that no cars were coming and that John is able to hoist a rattled detective up from off the ground in time.
"You're losing your vision," he murmurs, and Sherlock just nods mutely.
"Why didn't you tell me? I've been worrying about you for weeks."
"I … couldn't." The undercurrent of defeat in his voice negates any further questions. John hails a cab and leads Sherlock to it, pulling him away from the white car he is about to break into.
Night falls and with it come shadows. Sherlock lies down on John's bed, feeling more a ghost than a guest in this space, his only friend's final sanctuary against his presence. John sits down beside him and rubs his back as he begins to finally cry, the great wracking sobs that he has wanted to unleash for so long, that he has been hiding inside himself since when he first noticed that he could no longer see the difference between the stripes on his scarf.
He reaches his hand up to John's face and feels that he is crying too. With his lips he tastes the salt of his best friend and closest companion, pressing his fingers into his mouth and absorbing the intimate saline of their shared grief. Neither of them speak. The weight between them is too heavy for words. They cannot pretend that this is fixable, that they can return to what was before. John gathers Sherlock into his arms and holds him there. A new equilibrium must be established. They must relearn one another through a darkening lens.
"John," Sherlock whispers.
The detective shifts in the doctor's arms, cuddles down and holds him tighter. "Tell me the colors."