Could have been a green Christmas. I've had many of those here in Canada in places where the snow is normally very deep in winter, so a green English Christmas is not at unbelievable to me, and neither is a warm one (the Christmas where appropriate attire was shorts and tee-shirts is still legendary in my family).

Ben sold me on the idea that it was winter  in the scene just before Mycroft says, "Your loss would break my heart," when Sherlock wraps his coat tightly around himself. We see him do that three times in series three, once in each episode, and I wonder if there's any significance to it, but that could be the subject of another thread.

Back to the dress code, we've never seen the green shirt before, so I am convinced that he went out and bought it just for the party. I love how he obeyed the letter of mummy's instructions, but not the spirit, going with a very dark colour. Just enough compliance for mummy to be pleased, but not so much that it looks like he cares about her silly little gathering.

Mary

"Green christmas", well.... we have it quite often around here. But then all the trees are green, too.
Except some bigger ones without leaves which appear in some scenes from time to time... and right now I start wondering if they've put them there to make it more believable. Also they seem to have used a colour filter of some sort.