"The past is a foreign country: they do things differently there"
-The Go Between, L.P. Hartley

Everything was grey.

The world was a swash of grey, the grey of the sky, the grey of the pavement, the grey of the buildings all lined up neatly and lifeless along the street. He could briefly remember living in colour before, but every time he remembered, he realised he didn't want to.
He didn't want to remember the flushed pink of adventure against pale, pale skin, or the blue blue green eyes that were full of intelligence and knowledge and pain and sometimes maybe even love. But most of all, he didn't want to remember the red. The red of blood against the grey pavement, the red draining away the flushed pink and clouding the bluebluegreen.

Sometimes, he thought, it was better to see in grey.

The day colour came back into John's life started off like any other day. His eyes opened, his leg ached, his mouth ate and drank and pretended to smile, and then his body sat in his chair with a gun in his hand and didn't move, 'Today? Not today' echoing in his mind. What changed, however, was the knockknockknock on his door. While it often happened in the days and weeks after, it had been a long while since people had visited to make sure he was still alive and hadn't stuck a bullet in his mouth. Stuffing the gun back under the cushion of his chair, he forced himself to get up and politely tell his visitor to go away. However, as he opened the door, he didn't notice the grey of the wallpaper or the grey of handle, but instead saw nothing but white. The white of his skin, the white of his eyes enveloping that bright bluebluegreen but most of all, the white of pure, unadulterated shock. The shock, however, quickly morphed into the blinding white fury of rage, because of course, of course he would do this, of course he would take all the colour out of his life then come home back like nothing had changed, like he hadn't spent the last three months of his life aching for the vibrant colour that had ended the day it had stepped off a building.

John closed the door.

For the next few days, John saw only black. Staring at Sherlock's black leather chair, he remembered the black of his hair, the black of his stupid long coat and the black of the grief that had filled his lungs and his heart and his soul as he stood in the cemetery, noticing for the very first time the bleak grey of his tombstone.
A week later, he heard the familiar knockknockknock again and opened the door. This time, he allowed himself to look. He saw what he'd missed last week, how Sherlock's arms were tense by his side, how his chin was not as high as it should be, and as he opened the door wider to let him in, John noticed the slight limp that he tried to hide. Sitting down in his chair, he stared.
"This doesn't make anything okay" he said.
"I know. I just..." Sherlock didn't finish the sentence.
"Why?"No response.
Sighing, feeling 20 years older than he was, John stood up, and started walking towards his room. "Your room hasn't been touched" he said, hoping Sherlock heard what he couldn't say aloud.

Hearing him slowly limp towards the room, John knew he did.

John woke up with the horrible taste of grey in the back of his throat, the bleakness of his nightmare chasing away the dreams of pink and white and bluebluegreen. Gasping for breath, John slowly looked around the room, noting the soft yellow light filtering through his curtains and the plethora of colours vying for his attention, the residues of grey fading away as each second passed.
The abrupt sound of the kettle being turned on in the kitchen startled him from his lethargy, and he allowed himself to smile as he started preparing for his day. Moving into the sitting room, he sat on his chair, the sounds of Sherlock making tea washing over him. Looking outside the window, he marvelled at the healthy green of the trees, before he frowned to himself in confusion. Something felt off. As he sat there, he suddenly noticed the absence of the familiar solidness of his gun under his seat cushion. Standing up, he scanned the area around his chair, wondering if it had fallen out from it's place.
"Looking for something?"
John quickly looked up, and saw Sherlock calmly sitting in the kitchen looking at him intently. He noticed the green of the walls, the green peeking out from those bluebluegreen eyes, and as he was filled with the soothing green of peace, John smiled, looked into those eyes and said "No. I think I'm good."

John didn't know what it was that woke him up, but he quickly found he couldn't fall back asleep. Staring into the comforting cocoon of darkness, John lay still, marvelling at the quiet hints of colour that he had forgotten existed slowly revealing themselves. Had the wallpaper always been that comforting shade of blue, the blankets the earthly shade of green? He felt in his chest a peculiar lightness that had been missing for the past few months, and felt a sudden longing to see Sherlock, to make sure he was really there, lying in a bedroom next to his and not in a cold, damp grave.
Sitting up, he slowly put his slippers on, before quietly slipping out of his room and treading towards the room he hadn't had the courage to enter since before. He softly opened the door, and looking inside, John's heart was suddenly gripped with the poisonous hand of fear. The bed was empty.
Suddenly awake in a way he hadn't been since Reichenbach, his heart pumped loudly in his ears as the world became tinted in the flashing red of panic. Shouting out a frantic "Sherlock!?", he prayed to a god he hadn't prayed to in months that Sherlock was okay, that he hadn't imagined all the colour returning to his home and his heart.
However, as he approached the sitting room, he allowed his fear to subside as he noticed Sherlock standing calmly by the window, staring at his dusty violin in quiet contemplation. Suddenly embarrassed by his brief moments of panic, he quietly mumbled out "I didn't know where you had gone", staring resolutely at the faded red carpet.
Sherlock simply looked at him, before he picked up his violin (John was suddenly very glad Greg had stopped him from throwing it at the wall after) quickly blew the dust off and started playing a quiet melody. Staring at the soft glints of light shining off the motes of dust surrounding Sherlock, John looked at Sherlock's bathrobe, the dark blue echoing the sky blue of calm as he listened to Sherlock play his violin for the first time in months. Suddenly feeling the exhaustion of the past few days, John walked slowly to his chair before collapsing heavily onto it, the soft notes of Debussy guiding his way.

Several hours later, the soft orange glow of the sunrise found John fast asleep on the couch, Sherlock standing guard with his violin, both figures bathed in the ethereal light of peace.

"Why do you go away? So that you can come back. So that you can see the place you came from with new eyes and extra colours."
-A Hat Full of Sky, Terry Pratchett

A/N:
Being an expert googler, I can confirm that the kitchen walls are green, the carpet is red and the descriptions of their chairs are accurate as well.
I'm not entirely sure where this idea came from, but I'm a sucker for colour porn and 'Sherlock comes back and John doesn't know how to react' fics.
I hope you enjoyed, please leave a comment! Criticism is welcome :)