Disclaimer: I do not own Harry Potter or any of the characters. That's all JK Rowling's. But am I jealous? Yes.
Regulus Black's childhood was never very happy. His earliest memory was of playing alone in his room. He remembers vividly, too vividly it would seem for something that happened so long ago, the dark green walls that gave him a deep sense of foreboding even then. There had always been something about Slytherin green that had always made Regulus feel uneasy. Perhaps it was the color of the buckle on the belt that his mother would lash against his brother's thighs and back, leaving strips of flesh that Regulus would sneak into his brother's room in the middle of the night to help clean and dress. Maybe it was the background of his great-great-great-grandmother's portrait that would croon at him every time he walked by and praised him for being such a good and obedient heir. The old crone knew he was the younger brother and that Sirius was the one destined to be the Patriarch of the Ancient and Most Noble House of Black. She simply didn't care. Though maybe he didn't dislike the color at all, at least until he came to Hogwarts.
The second he walked into the Great Hall for his Sorting, he felt as though all eyes were on him. A quick glance around the ginormous room told him that wasn't the case at all, it had only been Sirius staring at him. They locked eyes and Sirius shot him a crooked smile and a thumbs up. Regulus looked down quickly as though burned by the gaze of the brother who looked so much like him dressed in all red and gold. With that image seared into his mind, a traitorous thought raced into his head before he could stop it. What if he was Sorted into Gryffindor? Sirius would be so proud. A beat later he realized it wouldn't even have to be Gryffindor. Anything but Slytherin would do. Not Slytherin. Not Slytherin. Regulus was broken out of his trance when his name was called. He walked over to the stool as steadily as he could but it felt as though any second his wobbly knees would give out and he would topple straight over. He sat down on the stool lightly but he was still shaking. He met eyes with Sirius again, this time his brother sent him no encouragement except bright hope shining from his gray eyes. Not Slytherin. He had seen his brother's eyes shine like that before. The woman slowly lowered the Hat onto his head. When his mother had been beating him the first time he had come home from Hogwarts wearing his Gryffindor scarf. Regulus' resolve crumbled at that thought. He would be shunned from his family home. He would be beaten mercilessly. His brother nervously tapped his knuckles against the Gryffindor table. He would lose his brother if he were Sorted into Slytherin, he was sure of that. There would be no shoring of that divide between siblings. Regulus made his choice.
The Sorting Hat went over his eyes and he was engulfed in darkness. "Hmm," it whispered. Its voice was surprisingly hoarse and raspy. "Another Black. Where to put you?" he mused. "Slytherin," Regulus' heart skipped a beat, "is usually the first choice for your kind. Merlin knows you would do well there," Regulus thought he was going to be sick. "Then again, you are very much like your brother. More so than you think. Be brave when you go back home, little king. You're going to need courage." Regulus was confused for a moment but then he heard the Hat call out "GRYFFINDOR!" and his ribcage felt like it suddenly turned to ice and was poking at his lungs and heart. The Sorting Hat was taken off of his head and he saw the reaction his Sorting had garnered. The Slytherins were a mixture of pure disgust with sneering faces and open-mouthed stares. He knew what they were thinking: another traitor Black. His eyes were drawn to the Gryffindor table when he stood up and piercing ice melted. Sirius was cheering with a wide smile on his face and he was struggling to get out of his seat. Regulus felt steadier now as he walked towards the Gryffindor table. He kept his gaze away from the Slytherins and his head help up high in a mask of trademarked Black indifference. Sirius finally was free from his seat and he called Regulus over with frantic arms gestures and jumping. Regulus could not fight off the smile on his face when he came face to face with his brother. Sirius' chest seemed inflated and he looked so, so proud.
"I knew you'd do it, Reg. I knew they hadn't gotten to you," Sirius wrapped his brother in a hug. Regulus tensed for a moment and Sirius noticed. "You'll get used to the hugging," he whispered in Regulus' ear. He patted Regulus on the shoulder and turned to the group of people sitting with Sirius. "This is my brother Regulus. My brother, a Black in Gryffindor!" The group cheered, disregarding the teachers that had been trying to get them to quiet down for quite a long time now. Regulus laughed loudly. "Moony, move over!" Sirius sat back down and pulled his brother into the seat next to him. Regulus felt pride bloom in his chest, his brother was sitting next to him at a meal! On purpose! Sirius continued to boast about this newfound Black legacy, but Regulus barely listened. He could only pay attention to the proud way Sirius would look at him and tugged his closer with an arm wrapped around his shoulders.
Regulus knew the Howler the following morning would make history. He knew it would hurt when he had to go back home for Christmas. He knew he would never again be praised by that old portrait. He was fine with that. He didn't regret his decision in the slightest, not when Sirius was proud of him again. He looked around one more time, comforted by the red and gold. Maybe he was just glad he wasn't surrounded by that dark moldy green.