Regulus saw green, sometimes, when he would knock on Sirius’ door early in the morning and his brother would tell him to wait. When Sirius finally stepped into the hallway, carefully closing his door behind him, he’d pretend that modesty was the reason he’d denied Regulus entrance while dressing, but Sirius would always be missing a sock or some other such article of clothing. Its pair would be hopelessly tangled in the sheets, lost during a fury of activity Regulus shouldn’t imagine, but usually did. Invariably there would be a smudge of lipstick in a place Regulus shouldn’t have been looking at in the first place, but often did. Regulus wasn’t supposed to notice any of these details, but always did.
There was no need to wonder who Sirius was hiding in his room, and not just because it was the always same person. Regulus wouldn’t need to wonder, because he would have already knocked on Bellatrix’s room and found it empty.
Regulus saw green, because he knew that no matter how much the two claimed to hate each other, no matter how much of that claim was actually true, Bellatrix and Sirius would always prefer each other to him.