Would you care for a bit of context? There's this awesome piece of fanart, here: http : / / 29. media. tumblr. com /tumblr_lfk5qeQ3dC1qbmkneo1_500. jpg (eliminate the spaces, you know the drill). And I got the horrible idea to write the backstory for it. So there you go.
"I feel a bit stupid."
"No, no, you look perfect," Sherlock says, not actually looking, but John doesn't really mind because frankly the whole thing's extremely awkward. "Rouge your shoulders."
"What?"
"Rouge your shoulders. And your chest. You're too pale."
"Oh." John makes a face. "Remind me again why I'm doing this."
"Because our serial killer goes for male prostitutes. Rouge."
"Right, right, I'm rouging." He opens the jar and starts to spread pink on his shoulders in little puffy clouds, hoping Sherlock actually asked before he pinched it from Mrs. Hudson's room. Probably didn't. Probably Mrs. Hudson's going to storm in looking for it and see John balanced on the armchair in a lace teddy and nothing else spreading makeup on himself, and God knows what will happen after that. He puts the powder puff down and poses for a moment, letting the strap of the teddy slide down his arm. Oh, hello, Mrs. Hudson, just getting ready for my nightly. Oh, Mrs. Hudson, fancy seeing you, I was just telling Sherlock how funny it would be if you walked in. Oh, hello, Mrs. Hudson, d'you like what you see?
Which gets him laughing until he notices Sherlock's disapproving stare.
"You're not rouging."
"I am so - I'm rouging! I'm just getting in character."
One eyebrow arches. John stares back at him defiantly.
"I am. I can't just go blundering out there carrying myself like an ex-army man. I've got to look the part." He snatches up his jumper in one hand and throws it over his shoulder, shooting Sherlock a sultry look, then drapes it behind his backside coyly, wrist bent, posing on the chair again. "Excuse me, Mr. Serial Killer, if you'll just follow me around this corner where my friend is waiting with the handcuffs - "
"Don't be idiotic." Sherlock prods John's chest with his riding crop. "I've bought you a very nice lace garment so you can help me catch a dangerous man, not indulge your fantasies of transvestitism, now rouge yourself."
"My fantasies," he says under his breath, picking up the powder puff again. "Who was it knew exactly what size I am in women's underthings? - Yeah, I'm almost done, give me one minute. This thing's riding up my arse."
"It's supposed to."
"What am I going to wear over it?"
"Nothing. Come on, the taxi's here."
"Nothing?" John's voice comes out a little more indignant than he intends.
"You're a whore, you have a high level of comfort with physical exposure." Sherlock grabs him by his freshly rouged shoulder and drags him towards the stairs. "Good evening, Mrs. Hudson, we're just on our way out."
John manages to cover his groin somewhat with his jumper as he stumbles after Sherlock, casting an apologetic smile at Mrs. Hudson. "Have a nice evening in, Mrs. Hudson! - My God, she's never going to speak to me again."
"On the contrary, I think she'll have plenty to say," says Sherlock Holmes, shoving him into the taxi.