Maybe they would talk about it later, or maybe they wouldn't. Sirius wasn't particularly worried about it, they all had their secrets - especially then. And if Remus needed white trees and purple ornaments to actually see the colors well Sirius couldn't deny his friend that, even if Regulus would have allowed it.
He did have to laugh though when James seemed to pick up on all they were saying with the looks they'd been trading. "This is why I love you so much." He said grinning, from anyone else he likely would have hexed them for what James had said. He might share looks with Remus and even Regulus from time to time, but James understood him better than anyone else ever had.
"No, I don't think anyone needs details - there will be plenty of privacy charms and we'll all go to sleep happy tonight, and that's all anyone needs to know." Sirius cut in before Remus could actually respond to James suggestion that he share details.
"Oi!" Sirius protested brushing the flocking from his hair. "We're decorating the tree, not each other Moony." He warned his friend with a laugh. Where Sirius was more than happy never to hear any details about his brother's sex life, he didn't have the same issues with his own being mentioned and he chuckled when James teased Remus.